At work, our computers running AIX and X-Windows, I've easily done the "Formatting Hard Drive" prank. I supplied a "Cancel" button, but when you pushed it, it changed to "OK", then brought up a timer bar w/lots of disk activity. . . . . we all worked graveyard so it was a great way to wake up coworkers.
The funniest I ever heard of is an Urban Legend (so I don't take credit for it):
Guy got fired from some faceless corporation, so, what he does is:
1. Finds a a fax machine
2. Feeds it two or three sheets of black crepe paper taped together. When the top of the first sheet peeked out of the machine's out slot, he momentarily halted it, and taped it to the last sheet, forming an endless loop.
3. He then dialed his boss's fax machine.
So what happened is, the Boss's fax machine went through all its paper before someone noticed & managed to disconnect it. Each page was all black and wet with ink, there were hundreds of them, and they created one hell of a gooey mess. . . . .