When I was little I put a rubber band around the spray handle on the kitchen sink. My dad was running a little late, dressed in full suit for a business meeting, and got drenched. While he was trying to appreciate the nature of the prank, he was livid.
A friend at the time did saran wrap under the toilet seat, this is definitely one of the more evil ones.
Musically, one of my favorites comes from Pink Floyd's Syd Barrett.
This is from wikipedia:
There are many stories about Barrett's bizarre and intermittently psychotic behaviour, some known to be true. According to Roger Waters, Barrett came into what was to be their last practice session with a new song he had dubbed "Have You Got It, Yet?". The song seemed simple enough when he first presented it to his bandmates, but it soon became impossibly difficult to learn and they eventually realised that while they were practising it, Barrett kept changing the arrangement. He would then play it again, with the arbitrary changes, and sing "Have you got it yet?". Eventually they realised they never would and that they were simply bearing the brunt of Barrett's idiosyncratic sense of humour.