A log scanner that failed deploys on the word "error"
A deploy-time log scanner grepping for the substring 'error' matched a benign 'Sentry not configured' line and failed the deploy.
The safety net had a hole
The test, the audit log, the scanner, the CI run — covering less than I assumed it did.
A deploy-time log scanner grepping for the substring. The line it matched was the app correctly announcing that Sentry wasn't configured. Trivial, and the archetype of the class: a check whose false positives are indistinguishable from real failures trains you to ignore it, which is worse than not having it.