[zwieback]

For me the moment was a year or so into my first programming job, this was in the 90s. We were debugging some device driver issues and found that we'd made bad assumptions about the order in which things happened between our interrupt handlers, device driver thread and application threads. Once I took the time really mapping out what all can happen in an asynchronous environment and then setting up long-running stress test with multiple clients it clicked in place that you really have to continuously test your mental model against what could happen in the real world.