My grandfather has been toying with computers since the either the 70's or 80's. One old lady he knows isn't very computer savvy and she called him over there to fix something minor for her. He takes the taskbar, moves it from the bottom of the screen to the side, and makes it span nearly half the screen. He calls her in there asking what has gone wrong with her computer. She covers her mouth, looking horrified, and says "Oh, I don't know..."
He giggled and showed her how to put it back.
He also taught my mom a prank to pull on the teachers when computer classes first came to her school. She called a teacher over to troubleshoot her computer. One button put "ow, don't push my buttons so hard", another put "why you so mean", and so on, onto the screen. Teachers hadn't the slightest clue why Mom's computer was on the fritz.
Some of my friends ganged up on one guy. They took a screenshot of his desktop, removed his icons, and put the screenshot up. They said it took him a while to fix it.