Most payment integration bugs never show up in a manual test. You click through the happy path once, it works, you ship it. Then in production a network blip causes a retry, two webhook deliveries arrive out of order, or a duplicate request slips through — and now you're debugging a double payout at the worst possible time. 63% of async API failures stem from race conditions invisible to synchronous testing tools. Payment APIs are almost entirely asynchronous — a transaction returns PENDING ...