I learned one of my most important distributed-systems lessons the hard way. We were working on a payment flow connected to an external payment gateway. On paper, the architecture looked solid: microservices, clean database transactions, retry logic, monitoring, and enough security checks to make us feel safe before deployment. Then production reminded us that real users do not live inside clean architecture diagrams. Support tickets started coming in. Some users could not complete payments. Some accounts were being blocked too aggressively. At first, it looked like suspicious behavior: multiple payment attempts, repeated payloads, and requests arriving only seconds apart. But when I dug into the logs, the real problem was not fraud. It was our own backend. A user with a slow network clicked the Pay button, waited, saw nothing happen, and clicked again. In another case, the browser retried a request after a timeout.…