The ones that spring to mind for the office (or perhaps at home too).

- we stuck sellotape under a lasermouse, over the red laser beam. The mouse doesn't work then

- we removed the mouse ball, and stuck paper inside the mouse, replaced the mouse ball, but when you try to use it - the ball doesn't move, but the mouse looks OK

- we disconnected one part of the telephone handset on the phone, so the cable LOOKS like it is connected. When the phone rings, you pick it up and the person on the other side can hear you, but you cannot hear anything

- we swopped the "a" and "s" keys, or the "H' and "J" keys on a keyboard. So when you type "handset" you get "jsndaet". Too funny that one!!! People have no idea.

- we used clear masking tape to stick a coffee cup down onto the desk. Surrounded the cup with paperwork, and when the person went to pick up their cup, nothing happened.

- change the phone ring on a cell phone. So when it rings, the person has no idea it is their phone ringing ......

- also, changed cell phone to "silent". (that is for those annoying people who always leave their phone on the desk to ring, and ring, and ring)