Hmmm. Nothing too great. One time we did the old swap keyboard trick. Really messes people up. Another time we photoshopped a colleagues head onto a certain adult picture (amazingly similar to what happened in The Office actually with a picture that sounds identical) then emailed it around the company. Erm, oh and we intercepted a fax to a work colleague from his mum on his birthday. It said things like "my special little boy" and stuff so we modded it to have things like "lovely little soldier" and the like then stuck copies up all over the office. Heheheh, good old days when bosses had a sense of humour
My favourite though was when we put a program into memory back in about 1989. It was one of those memory resident programs that I forget the name of (if anyone remembers it was like Sidekick that appeared at any point when you pressed a key or two). Anyway this one was called Monster and if you pressed the letter M then it would fill the screen with an ASCII monster. A girl did an entire 10 page assignment without using the letter M
My favourite prank that I wished I had done was when a friend of mine got kicked out of college after a year. In response he modified all the boot disks in the main programming room and when the new first years arrived and booted up the whole room was full of hardcore porn videos. Pretty good for 1990
Cheers
Oh, and once we took a colleagues bunny toy and hid it. A friend then took it round Europe and sent back pictures of it in different places. When we got it back we bought an identical one and cut it up, covering it in fake blood etc and gave it to her as a secret Santa present

Was amusing how many people didn't want to cut up the copy cos it was so cute
I am 99% probably lying in bed right now... so don't blame me for crappy typing
Current fave quote : "She was like a candle in the wind.... unreliable...."