Behaviour changes are test-first
Do not change behaviour on the strength of reading the code.
- Write a test that exercises the specific code and fails because of the problem.
- Run it and see it fail.
- Make the change.
- Run the same test and see it pass.
- Report both runs.
A suspected cause that no test can reproduce is still a hypothesis; say so instead of changing code.
Tests must not depend on a harness account, a model, the network or timing, so that they also pass in CI.