My favorite has to be a prank an old timer mechanic pulled on a salesman when I was 18/19 working at a Dodge dealer. I think it was on a Jeep Grand Cherokee. You filled the rear washer fluid near the rear tail light. He asked the salesman to watch the tail light and let him know when it was full. After a minute he finally figured it out and told him to go #@#@ himself.

We all laughed our asses off.