FM8 — Schema/Contract Violation: The primary failure mode that testing strategy exists to prevent. Contract tests catch these before deployment. End-to-end tests catch them after deployment to a test environment. Without any contract enforcement, schema violations reach production.
FM11 — Observability Blindness: A test suite that tests only happy paths provides false confidence. When the production system encounters an error case (timeout, partial failure, invalid input), the untested code path runs for the first time in production. Testing strategy must include explicit coverage of error paths: what happens when the database is unavailable, what happens when the external API returns a 500, what happens when the input is malformed.