I always screw around with friend's profiles when they leave them open, making them appear gay and psycho. They're always mad at first, but eventually they calm down enough to actually realize how funny it is.
In Graphic Design we used to do that, if we came in and the person in the previous period didn't log out we'd put a gay desktop picture and rename files and stuff, some pricks deleted the person's files but I never went that far.
Oh yea we also rearranged all the keys on the keyboards, like one keyboard had 6-6-6 6-6-6 and forget YOU on it and the teacher came to us the next day, being the cigarette hippy she was she said "Did you guys have an artistic break-out on the keyboards yesterday" and we said "yes" and she just left. (BTW to get forget YOU with 2 Us we were jacking keys from a ton of keyboards.)
