I love playing little pranks. My best one was when I worked at Chautauqua County as a database programmer/network administrator. I made it so when people logged in, it said in giant letters:
WARNING! Your files will be deleted.
Press any key to continue…
They had to press a key 3 times and then it scrolled up, in ASCII art text, April Fools!! Only one person really panicked and most thought it was funny so I guess it was OK. Tee hee! :>