Stuff like that was pretty funny at school. 90% of the guys in my class barely know how to turn on the computer, so it's pretty easy to fool around with them. When we were using computers we send them all kinds of fake errors via net send cause the pop up window looks like the usual error windows (Windows NT...urks)
"Like Error 404 - Impotent User"
Took them weeks to notice we were faking it...
Espacially funny if you watch the guy (sitting behing him or so) and always send an error when he does a certain thing. You can really make them belive that the right mouse button doesn't work and stuff.
Yay...good old days
Also the Windows RG flash thingy has some good errors.
"Word has to be closed (send to prison) for performing an illegal action (killing a paperclip).