Users kept reporting something was "off." No error. No trace. Nothing in the logs. We spent days looking at the system. Wrong move. The moment we switched to watching how people actually use the product — tabs open, sessions overlapping, mid-flow refreshes — it clicked. The bug was never in the code. It was in the assumption that users behave like the happy path in your tests. They don't. They multitask, they wander, they come back 10 minutes later. Made critical actions idempotent, removed state dependencies, added guards for parallel execution. Bug gone. Assumption updated. Write code for the user you have, not the user you imagined.