Roblox Failure Archaeology

Roblox Failure Archaeology

Overview

Canonical case studies from shipped firebit games. Each one is a battle that was already fought and won; the scar tissue (odd-looking code patterns) is load-bearing. Check this chronicle before diagnosing a familiar-looking symptom or "simplifying" a pattern that exists because the naive version failed. Every case study follows the same shape: symptom → root cause → mechanism → generic lesson → how to detect the same class in your project.

The concrete incidents below come from a shipped firebit game (the reference implementation of the house stack); the lessons are stated generically and hold for any project on the stack.

When to use / When NOT to use

Use when:

• A bug symptom matches anything in the index table below — read that case study before touching code.
• You are about to change model-placement math, NPC pathfinding, animation lifecycle code, spawn flow, or client tag-handlers — these areas are minefields with documented history.
• You need to know whether something is a bug or a deliberate design decision.
• You are setting up or updating the project's own OPEN-bugs register (format at the bottom).

Do NOT use when:

• You have a live symptom and need a triage procedure → use roblox-debugging-playbook (symptom→experiment view of this same material).
• You need the rules about what changes are allowed → roblox-change-control.
• You need architecture invariants (why services are shaped this way) → roblox-architecture-contract.
• You need Roblox platform theory in general → roblox-platform-reference.

Index

Case studies

1. Placement rotation saga (the crown jewel)

Shipped firebit game, 2026-05/06: ~15 commits over a week to place a trap model facing the right way.

Symptom (evolving): placed models appeared rotated 180° from the placement ghost; then descendants of multi-part models were mirrored while the PrimaryPart looked correct; then the whole model sat at the asset's original CFrames despite a "correct" pivot.

Root causes, in discovery order:

1. Placement was routed through a generic loot-spawn helper whose bounding-box part drifted orientation for asymmetric assets.
2. Client and server each derived the CFrame independently; they drifted.
3. Left-handed basis + silent network normalization. up:Cross(WORLD_FORWARD) yields a left-handed basis. Roblox's CFrame network serialization silently normalizes to right-handed (flips RightVector). The client's copy crossed the network and got flipped; the server's identical local math did not — a 180° disagreement invisible in same-machine tests. CFrame.fromMatrix on the ghost side did the same silent flip, mirroring descendants.
4. PivotTo on an unparented Model can leave descendant parts at their original asset CFrames while only the WorldPivot reference updates. Parent/pivot ordering was flipped twice for different reasons — read the history of that ordering in your project before touching it.
5. Euler convention mismatch: CFrame.Angles composes XYZ; it does not round-trip values from ToEulerAnglesYXZ. Use CFrame.fromEulerAnglesYXZ to match.
6. Asset authoring: some models shipped with WorldPivot offset 180° from the PrimaryPart.
7. Sending basis vectors over the wire invited all of the above.

Mechanism: every one of these is a representation mismatch between what one side computed and what the other side received or applied — the engine "helps" (normalizes, re-orders, defers) exactly once, on exactly one side.

Final architecture (the settled pattern): client computes a right-handed frame by construction (WORLD_FORWARD:Cross(up)) and sends position + a { rY, rX, rZ } table from ToEulerAnglesYXZ; the server reconstructs via CFrame.fromEulerAnglesYXZ and applies PivotTo with a deliberate, documented ordering relative to parenting.

Generic lessons (each cost real debugging days):

1. Never send raw CFrames or basis vectors client→server. Position + YXZ Euler angles, reconstructed identically on both sides.
2. Any hand-built basis must be right-handed by construction — never rely on engine normalization, because it only happens on one side of the wire.
3. CFrame.Angles ≠ fromEulerAnglesYXZ. Match the extractor to the constructor.
4. Check PivotTo-vs-parenting order and the asset's WorldPivot before suspecting your math.
5. When client and server disagree, log both sides of the wire — the saga burned days assuming the received value equaled the sent value.

Detect the class: grep your action payload definitions for CFrame-typed fields (grep -rn "CFrame" src/Shared around network/action type definitions); grep for :Cross( and verify handedness at each construction site; grep for CFrame.Angles( fed from ToEulerAnglesYXZ output.

2. NPC swarm freeze — six stacked causes

Shipped firebit game, 2026-06: zombies circled, froze, or wandered instead of reaching the defended object. All six causes fixed in one commit.

Root causes / mechanisms:

1. NPCs shared a collision group but self-collision was never disabled → bodyblocking (pathfinding ignores dynamic bodies). Fix: disable NPC-vs-NPC and NPC-vs-player collision at collision-service init.
2. Chase logic re-issued walk commands every 0.5s at a stationary target, tearing down the movement library's follow loop before the NPC advanced. Fix: re-issue only on LOS loss, destination drift, or measured stall.
3. The pathfind budget queue was dead code (its pending-target field was never set) → every chase hit PathfindingService directly → a spawn burst flooded ComputeAsync → throws → frozen NPCs. Fix: actually route through the queue, dedupe queued entities, run walks detached via task.spawn.
4. Fresh spawns idled while queued. Fix: immediate straight-line walk first; the queued path refines.
5. A set-all-targets call only mutated existing NPCs; future spawns reverted to the default mode. Fix: store the mode and apply it on spawn.
6. Small destructible targets were unreachable (tight bounding box + short attack range). Fix: an explicit reach-clearance constant.

Generic lessons: budget and queue all ComputeAsync calls; never re-issue movement to a target that hasn't moved; disable NPC self-collision; global mode setters must also apply to future spawns.

Detect the class: grep for direct PathfindingService / ComputeAsync calls outside the one queued chokepoint; grep for periodic WalkToPoint/MoveTo re-issue loops with fixed timers; check CollisionService-equivalent init for NPC-group self-collision.

3. Players spawn at world origin

Shipped firebit game, 2026-06.

Symptom: everyone spawned at (0,0,0); a place variant also hung on load forever.

Root cause / mechanism: CharacterAutoLoads=true in the place project.jsons with no SpawnLocation → the engine pre-spawned everyone at origin, and an if not player.Character guard then skipped the custom spawn flow entirely. Separately, one place variant waited on a readiness signal that that variant never fires.

Generic lesson: the house convention is CharacterAutoLoads: false in every place project.json — a player service owns spawning via tagged Spawn parts (see roblox-architecture-contract). And never :Wait() on a signal without confirming every place variant fires it; gate variant-specific waits behind a place check.

Detect the class: grep -rn "CharacterAutoLoads" *.project.json (must be false everywhere); grep :Wait() call sites and confirm the signal fires in every place that runs the code.

4. Tag-added handler saw partial replication

Shipped firebit game, 2026-06: placed multi-part trap models had no pickup prompt.

Root cause / mechanism: a multi-part Model's CollectionService tag replicates to the client before its child parts and their tags stream in. A one-shot resolve inside the tag-added handler returned nil and never retried.

Generic lesson / pattern to reuse: every client CollectionService tag-added handler must tolerate partial replication — bounded poll for the expected descendants (bailing if the instance is destroyed), with a pending-set to dedupe duplicate waiters. Never a one-shot lookup.

Detect the class: grep client code for GetInstanceAddedSignal / tag-added handlers and check each for a one-shot FindFirstChild on descendants of the tagged instance.

5. FireClient on nil after a yield

Shipped firebit game, 2026-06: leaderboard service crashed when a player left mid-update.

Root cause / mechanism: a yielding call inside a per-player loop; the falsy-return path used continue and skipped the player-left check, so a player leaving during the yield nilled the player-keyed cache entry and the subsequent FireClient indexed nil.

Generic lesson: after any yield in a player loop, re-validate or pre-capture player-keyed state (capture the cache table in a local per iteration). Every code path after the yield needs the check — including early-continue paths.

Detect the class: in server loops over Players:GetPlayers(), find yielding calls (GetValue, DataStore, WaitForChild, Promise awaits) and check what player-keyed tables are indexed afterward.

6. Wrong vendored-package API, twice

Shipped firebit game, 2026-03 and 2026-05: Maid:GiveTask / DoCleaning shipped broken twice.

Root cause / mechanism: the house stack vendors dig1t/maid, which exposes Add / Clean / Destroy — NOT the quenty-style GiveTask / DoCleaning that models commonly assume. Wrong method names only fail at runtime.

Generic lesson: verify vendored package APIs by reading the source in Packages/ before calling — never trust model memory for library method names. This holds for Maid, Signal, Promise, and every other Wally dep.

Detect the class: grep -rn "GiveTask\|DoCleaning" src/ — any hit against dig1t/maid is a latent runtime error.

7. NPCs spawning on/in the defended object

Shipped firebit game, 2026-04: zombies spawned on the RV roof.

Root cause / mechanism: the spawn-position raycast could hit the protected object itself and accepted candidates arbitrarily close to it.

Generic lesson: exclude the defended/protected object from spawn raycasts and reject candidates below a minimum XZ distance from it.

Detect the class: read the spawn service's raycast params — is the central gameplay object in the exclusion list, and is there a min-distance check?

8. ViewModel physics chaos (ported code)

Shipped firebit game, 2026-03/04: a first-person weapon system ported from another project fought Roblox physics for weeks.

Generic lessons: Studio-vs-live replication differences are a recurring class — always suspect them when "works in Studio". And idempotent-looking per-frame state sets are not idempotent for animations.

Detect the class: find per-frame (RenderStepped/Heartbeat) callbacks that set state unconditionally rather than on change; check any camera-attached rig for anchoring and a dedicated collision group.

9. Avatar Joint Upgrade migration — settled recipe

Shipped firebit game, 2026-05: Roblox's Avatar Joint Upgrade (Motor6D → AnimationConstraint) broke weapon animations. The fix is a complete, verified recipe — apply it, do not re-derive it:

• Stop destroying the default Animate LocalScript (client and server).
• Force IsKinematic=true on every created AnimationConstraint; catch late additions via DescendantAdded.
• Convert weapon-model Motor6Ds to kinematic AnimationConstraints on server init, preserving rest pose via Attachment.CFrame = Motor6D.C0/C1.
• One-heartbeat Attachment "poke" under HumanoidRootPart on spawn/equip (per a Roblox staff workaround) to re-wire nil-attachment constraints.
• Zero-weight all Core/Idle/Movement tracks on equip (restore on unequip); tool tracks at Action4 priority.

Generic lesson: platform migrations of this kind get one reference implementation per studio; new projects copy the recipe, they don't rediscover it in production.

10–12. Tool/item lifecycle bugs

• Animation races — a play racing an unequip left a stray idle track → generation counter on play/stop, so a stale async completion can't act. LoadAnimation leaked a track per call → cache tracks per animation id. Rapid replay faded against itself → Stop(0) + TimePosition reset.
• Attack dead after equip+unequip — stale active-tool state never cleared; gate on the client's authoritative hotbar/slot state. Sibling bug: a server-side looped hold AnimationTrack kept replicating after unequip until explicitly :Stop()ped before destroy.
• Dropped items became grey cubes, dead on pickup — three stacked causes: the item's model field unset (fell back to a plain Part), the loot pipeline collapsed multi-part Models to the PrimaryPart, and the pickup path dropped the props metadata so the item's identity didn't survive re-equip. Generic lesson: an item's model + props metadata must survive the entire drop/pickup pipeline; test the round trip, not each half.

Detect the class: grep animation code for LoadAnimation calls inside handlers (should be cached); grep equip/unequip paths for state fields that are set but never cleared; round-trip-test drop→pickup→re-equip for every item category.

13–15. Smaller settled fights

• Placement ghost climbed invisible sensor volumes — the placement raycast hit another system's invisible collision-group volumes. First hack reused an existing group; the proper fix was a dedicated collision group for the new raycast system, registered in the shared collision-groups data module. Generic lesson: new raycast systems get their own collision group.
• AoE trap didn't hurt its triggerer — a tight-radius spatial scan can miss the entity standing exactly on the origin. Generic lesson: always union the triggering entity into the AoE victim set.
• Mobile infinite yield — WaitForChild on a legacy ScreenGui element absent on mobile hung forever. Generic lesson: unbounded WaitForChild on legacy GUI is banned; pass a timeout or (better) port to React (see luau-react).

Detect the class: grep -rn 'WaitForChild("' src/Client | grep -v "," — hits without a timeout argument on GUI paths are hang candidates.

16. Data reorg require breakage

Shipped firebit game, 2026-04: reorganizing shared data into feature subfolders needed three same-day follow-up fixes.

Root cause / mechanism: relative requires missed in the sweep, plus one auto-generated file that had to move back because its generator writes to a fixed path.

Generic lesson: folder moves require a full relative-require sweep plus a check for generated-file path assumptions (rblxsync's Generated.luau, asphalt image indexes, sourcemaps). Generated files stay where their generator writes them. Regenerate the sourcemap after moves (npm run build:sourcemap).

Detect the class: after any move, run luau-lsp/the analyze hook over src/ and grep for the old path segment.

17. Require cycles

Shipped firebit game, 2026-06: two service require cycles.

• One cycle (three services in a loop) broken with (require :: any) on a single edge.
• Another broken by provider injection: instead of requiring the other service, inject a small getter function (e.g. getCurrentDay) at init.

House pattern: break cycles by provider injection where practical; (require :: any) on one edge is the fallback. Same cleanup surfaced two Luau-solver lessons: direct table iteration yields a distinct metatable type (wrap in pairs()), and the solver won't narrow T?→T across tuple-assign (use fresh non-optional locals) — see luau-type-expert.

Detect the class: luau-lsp reports the cycle; grep for (require :: any) to find existing deliberate breaks before adding requires between services.

Design decisions — do NOT "fix" these classes

D1. Deliberately scaled-down automated-kill rewards. Shipped firebit game, 2026-07: kills made by placed automated devices credit the placer but pay reduced XP, so "place device, go AFK" cannot out-earn active play; a direct player finishing blow pays full reward. Generic lesson: reward scaling on automated/passive sources is anti-AFK economics, not a bug. Any change is an economy-tuning change → gates in roblox-change-control (hard rule 4: rationale + predicted metric + evidence path).

D2. Legacy live products intentionally undeclared in rblxsync.yml. Shipped firebit game: carryover developer products from an earlier era of the game are deliberately NOT declared in the manifest; they live on Roblox untouched until explicitly deprecated. Declaring them risks case-sensitive-name mismatches minting duplicate live products (roblox-change-control hard rule 2: never rblxsync run live; validate/--dry-run only). Related: flag-gated dormant code from earlier eras is fenced by hard rule 3 — leave it alone.

Generic lesson: before "fixing" an apparent inconsistency in monetization manifests or reward math, check the manifest comments, the project's CLAUDE.md, and docs/plans/ for a recorded decision. Absence of a fix is sometimes the fix.

Maintain your project's OPEN register

This skill deliberately carries no live bug list — open bugs are project state, not studio knowledge. Each project MUST keep its own register, either as docs/plans/open-bugs.md or inside a project-local skill. Required format per entry:

### O<N>. <One-line symptom> — <STATUS: reported | verified | diagnosed | planned>
- **Evidence:** <file path + what to look at; plan file if one exists>
- **Verified:** <date it was last confirmed still open>
- **Re-verify:** `<one-liner command whose output proves it is still open>`

Rules for the register:

• Every entry needs a runnable re-verify one-liner (e.g. grep -c '\- \[x\]' docs/plans/<plan>.md where 0 = still open, or a grep -n on the offending line).
• When an OPEN item gets fixed, move it to a settled note in the same change as the fix. A register that disagrees with the repo is a bug.
• Live debug flags, unseeded Random.new() in generation code, and ungated print spam are register-worthy classes (learned in a shipped firebit game: a committed debug flag distorted every structure-distribution observation, and diagnostic prints committed during debugging outlived their fixes). Sweep for them: grep -rn "DEBUG_" src/Server, grep -rn "Random.new()" src/, grep -rn "^\s*print(" src/Server.
• Economy-touching open bugs (double payouts, reward miscounts) need their own plan before fixing — roblox-change-control hard rule 4 applies to the fix too.

Common mistakes

• Re-deriving a settled fix. The Joint Upgrade recipe, the placement wire format, and the pathfind queue encode hard-won platform knowledge. Read the case study before rewriting the area.
• "Simplifying" scar tissue. Bounded polls in tag handlers, per-iteration cache captures around yields, Euler-table wire formats, immediate-straight-line-then-refine spawn walks — each looks over-engineered and each replaced a shipped bug. If a pattern looks weird, search this chronicle and the project's git history before deleting it.
• Treating design decisions as bugs. Scaled automated rewards and undeclared legacy products are decisions with rationale. "Fixing" them without the roblox-change-control gates is a regression, not a cleanup.
• Citing evidence from memory. During the original compilation, a commit hash cited from memory was one character wrong. Verify claims against the current repo (git show --stat, grep -n) before citing them.
• Assuming Studio behavior equals live behavior for replication order, camera-rig positioning, or CFrame serialization. Three separate case studies (4, 8, and the saga's handedness flip) were invisible or different in Studio.
• Fixing an economy bug as a drive-by. Anything touching live payouts needs its own plan, rationale, and evidence path per roblox-change-control hard rule 4.

Provenance and maintenance

Derived on 2026-07-05 from the firebit house-stack reference project (git log/git show --stat over the full history plus direct working-tree reads; every underlying commit claim was individually verified that day). Commit hashes and repo-state specifics were dropped in this generic edition; attribution dates in the case studies are commit dates from that verification.

All re-verify and detection commands in this skill run against the current project repo (repo-root-relative, per the house structure: src/Client|Server|Shared, Packages/, docs/plans/, *.project.json). If your project deviates from the house layout, adjust paths — do not skip the check.

Maintain this skill by adding new case studies in the same symptom → root cause → mechanism → generic lesson → detection format, with a game/date attribution line. Project-specific open bugs go in that project's OPEN register (section above), never here.