This was done to me....too funny.
Go to someone's computer that's running (ensuring there are no open files where data might be lost), and type "Format c:", then unplug the keyboard and mouse. The persone comes back to the computer, and on seeing the command, immediately tries to backspace, wiggle the mouse, etc....with NO results! They call the tech guy over, and he does the same, while cursing that he'll have to load all the software over, how can people be this malicious, etc etc. Of course, they turn off the computer and restart it to see how much damage was done, and they get the same dead mouse and keyboard. After about 30 minutes of monkeying around, they finally figure out the mouse and keyboard, but it might take some explaining to enlighten them as to the whole gag. Too funny!