and when you really want to mess with someone, pop the keys off the keyboard and move them around so they aren't in the right place. If you really hate them, move them all 1 place to the left or right so that it's not immediately evident what's happened.
Can also be done with certain calculators. Switching the +, - , X, / keys is most effective ;-)
...not that I know from first hand experience
...I just imagine it could be done like that... say to a bud in grad school who went to the restroom for a sec...
I'm just sayin'
Pulling the anti-theft strips from a library book and hiding it in a friend's backpack binding is also something that could be done....
....or so I would imagine......