A business customer and I colluded to get the salesrep from our company on April 1. The customer called the rep in the morning to complain that the system I had implemented several months earlier was acting up and not printing the results correctly (medical testing information). Calls went back and forth all day, with the customer becoming more agitated and the rep getting more frustrated with the lack of resolution from me.
Finally we set a conference call for late in the day. The rep tried to be the middle-man, brainstorming on all the things that might be wrong. My responses to his suggestions ranged from "I don't know" to "That might be the problem but I doubt it", etc... generally, I was no help at all. The customer continued to sound more frustrated and you could hear the rep really trying to control himself. Finally, the customer asked me "Do you think it might have something to do with the date?", and I said "You mean the fact that it's April 1st?".
About 15 seconds passed in silence before the rep realized what we had done. It was classic.