Of all the April Fools pranks that I came across today, the Stealth USB CapsLocker was my favorite. The tiny AVR-driven USB device sends random caps lock keypresses to a PC via a USB interface. The user will see their caps lock light come on from time to time and think they’ve accidentally hit that most useless key on the keyboard.
Then they might see the Caps Lock light turn on by itself. Next is a sequence of reboots, bashing the keyboard on the desk, clicking through the Control Panel, possibly even replacing the keyboard. Unless they notice the tiny little device sitting in one of the USB ports on the back of their computer, nothing will help.
Equally as cruel, but slightly less technical, would be to switch someone’s keyboard mapping to be like the old Sun keyboards (with the control and caps lock key positions swapped).
Be careful, though. There might be some cranky old unix guru who actually appreciates this configuration.