Some of the time its a case of even identifying a mistake has been made or a fault has occurred.
Had one recently - random systems had the same type of configuration issue crop up. The issue was similar in nature. It was fixed on each occurrence and the world kept of turning. I just happened to notice the pattern no-one else did (and that was only because I was cc'd on email - I wasn't working the problem), tracked it back to someone going around a list of systems and performing some work with an unexpected side effect (rather ironically the work being performed was tested, but the issue was not identified as it matched the test system's configuration).
Mistakes will be made. Faults will occur. The important thing is to learn from them (at least in my opinion).