One of my first jobs in design was working on a match program for a certain well known football team. Across the bottom of the main pages in between the folios, there was a message that said 'To hold your own private party at the satdium, call XXXXXXXX now'.
When I nipped to the loo one day, one of the other lads deleted the y.
A few thousand got printed and there were no end of recriminations, but luckily I kept my job after claiming (rightfully) innocence.
When I found out who it was, I got my own back in typical early twenty something style by nailing his fiancee just before I left for a new job.