Well, since we're on simple hardware pranks...

Back in high school, I thought it was hilarious to swap around the numerals on the keyboard. My two favorites were: moving 0 to the beginning of the number row and shifting the remaining keys over by 1, and swapping the numpad to telephone order with 1 at the top left. Much more logical layouts if you ask me. Alas I don't think I got too many people with that one because our logins included our graduation year...