You might think that's just a joke, but it happened to me once.
I was working on some minicomputer system performace software and a
customer reported that the system totals for some of the measurements
were overflowing.
After looking at the code, the only way I could see that it could
happen was if some of the counters of elapsed time in different states
were running backward for some processes. I thought "no way", but
went ahead and put in a check for that condition to trigger a
debugging printf.
The next morning the boss let me know four things. First, I had
guessed right that the elapsed times were sometimes running backward.
Second, that I had left off an argument in the printf call, resulting
in a bad memory reference. Third, that I had neglected to switch the
processor back into user mode before calling the printf. And fourth,
that the customer would have preferred the original problem to having
their production system go down in the middle of the day.