No matter how hard you try, an occasional defect will escape to production despite your best efforts. Ideally, this is a rare event. When it happens, the defect is likely to be a devious problem that causes strange things to happen in other systems you didn’t know about during development. Because of a big security fence between development and production, you are unlikely to hear much about many of these defects, let alone participate in chasing them down and finding their root cause. Certainly not every operational software is caused by software, but a lot of them are. And when they are, the development team should be keenly interested in discovering what it was about its design and testing practices that caused the defect and allowed it to escape. In our experience, this kind of feedback is usually not sought and generally not welcome. This is a mistake. The only way to prevent escaped defects in the future is to examine the ones that occur in the present, methodically find their root causes, and, one by one, eliminate them.
Source: Leading Lean Software Development, Mary & Tom Poppendieck, Addison-Wesley