OK, another punch card story.
In the early days of SCCS, when it was a SNOBOL4 program running on an IBM mainframe, my officemate, Alan Glasser, and I provided tech support, and a user came to our office with a bug: It had processed the same card twice. SCCS listed all the commands on the printout, and, sure enough, the command had been processed twice.
Alan, who was smarter than me, had no explanation, and neither did I. I said to the user: "You know, there are all kinds of bugs, some pretty weird, but that just isn't the kind of bug that I would have. Just not possible."
Finally, I think it was Alan who said: "Bring us your deck."
The user returned a few minutes later, and we examined the deck. The command card was in there twice.
Nailed it, and it only took us an hour.
--Marc Rochkind