XXX should not be here
Back in the late 90s one of the guys working for me was having problems with a Perl script that did some sort of user facing system function - I don't recall the details. I did not and still don't do Perl, so I wasn't able to contribute anything to solving the actual problem, but I suggested he do what I typically do with debugging stuff that seems to be doing something "impossible". Insert some diagnostic messages.
He pointed to a particular function filled with Perl gobblydegook and said the problem has to be here, but the logic shouldn't allow it to get to where it was printing garage data. I grabbed the keyboard and added a line to print "XXX should not be here" and told him to run it again. He tried it with various inputs, and was eventually able to get it to print that message. He started in on modifying the print command I'd added to print the value of a few variables to get more information, meanwhile my phone rings.
It was one of the math professors, who says "the computer is accusing me of having porn". I said there isn't anything we are doing to detect porn, it might be sort of web ad but he insisted it was not an advertisement it was "us" telling him that. He wasn't the best at providing useful diagnostic information, obviously, so I walked down the stairs to his office and I see "XXX should not be here" on his screen running the software we were debugging. Apparently my guy was looking at the public copy of the script and didn't tell me that when I modified it, then the math prof used it at exactly that moment.
I explained to him what that meant, everyone had a good laugh, and we all learned to be more careful to insure we aren't modifying the public copy of something during testing/debugging (and to make sure I told him to take the file ownership of all those Perl scripts out of our admin group, so that we'd have to become root to update the public copy)