Beware of young children... and facetious colleagues. Have been the victim and perpetrator a few times. My favorite was to change the screen saver to a BSOD screenshot and watch the reaction of the colleague when he walks back to his desk.
True story: I once did this[0] to a friend of mine (who at the time was also my boss) on his Ubuntu desktop. I later asked him about it since he never said anything. Turns out he just thought his Linux Desktop was being stupid again, used the control sequence bring up the terminal, and just did his work in there.
[0] It was a little different. I set the screenshot as the background and hid everything and removed all panels and whatnot.
