As an example, I had a python function that needed to strip the front of a string and went into an infinte loop on an empty string due to a boneheaded mistake on my part. I failed to have a unit test for that particular input and didn't catch it till months after it went into production when empty strings started showing up occassionaly in the wild, and my program would get restarted by a watchdog which noticed it not making progress. A fuzzer would hopefully have caught that sooner.