summer camp, when I was like 14... while some kid that I hated was sleeping, I put toothpaste across his eyelids. the paste hardened overnight, and when he woke up in the morning, he couldn't open his eyes

best part about that one was that he was sleeping on a top bunk. he totally freaked out.
the big stage crew "prank" every year was to take the most annoying freshman stage crew member, tie them up with rope and duct tape, and then leave them in the girls bathroom for a few hours (I went to an all-guys school, so the female bathroom was pretty underused). we got in serious trouble for it during my junior year, though, when a teacher found the student, so that put an end to the tradition.
'bout 4 in the morning, junior year of college... I put a bunch of gay pride bumper stickets on a crapload of cars that were parked outside of a rival frat house.
a few months ago, one of my friends left me alone with their Dell for a few hours. I put in a Mandrake Move CD, set the BIOS to only boot from CD, and left the computer turned off. When she came back, I had her convinced that I formatted her hard drive and installed Linux over it. I let her believe that for a good 6 hours
