You write a record, immediately read it back, and assert equality. The test fails. Not because of a bug, but because the read hit a replica that hasn’t caught up yet. Your test is correct. Your assertion timing isn’t.

Team A changes their API response. Team B’s service breaks in production. The integration test suite passed because it was running against a mock from 3 months ago.

Your system passed all tests. Every health check is green. You’re confident it handles failures. Then a network partition happens in production and everything falls apart. You never actually tested failure.