Everyone who’s done computer pranks before has likely swapped out keys on their victims’ keyboard, or maybe even adjusted the key mapping so nothing they type comes out they way they wanted to.
However, something even more hilarious is plugging in a secondary keyboard to their tower. This is a fantastic prank option to play on co-workers in the cubicles around you, since you can wire the keyboard under the walls and desk without them seeing it.
All you need to do once you’ve plugged in the second keyboard is keep it at your desk, and start typing random letters, codes, or messages to the person on the other end. It’s especially useful to type when you hear them start writing.
If you keep it simple at first, they’ll think they simply mistyped. Then you can ramp your way up from there until they’re wondering if they have a virus or were hacked!