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Audit logging, validation, and mapping that were 'handled centrally' but only covered the endpoints that happened to route through the centre. On guarantees that are really conventions.
Three things that appear done and aren't: a log tailer that drops half a line, a timeout that doesn't cancel anything, and a process that ignores the polite request to die.
A test that only failed one run in five, on a component labelled deterministic, pointing at a hash that quietly reshuffles itself every time the process restarts.
Building a scanner teaches offence and defence at once: pick markers that can't happen by accident, score by how forgeable the evidence is, and watch your own outbound calls.
Every tenant-scoped system I've worked on had the same class of bug: isolation that looks centralised and airtight, with a handful of quiet ways for one tenant's data to reach another.
In a system where modules turn on and off at runtime, disabling one should never drop its tables. On reversible state, and giving each module its own migration history.
Two bugs that live in the first few milliseconds of a process: a security check that ran too late to matter, and a test that froze the logger before the app could build.
Two correctness bugs that never appear when everything goes right: a tenant filter that only works on the request path, and an aggregate saved in two steps that can strand half of itself.
You can't fix every latent problem at once. But you can freeze how bad it is and forbid it from getting worse. On turning architecture rules and scary refactors into tests that fail loudly.
A full green test run is not the same as working software. Two cases where every test passed and the feature was dead in production, and what the tests were actually failing to check.
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