We work in an office where we must lock our computer screens because our screens may have confidential information on them, plus you could do things as the person without taking any blame (ie sending an email).
Anyways anytime someone forgets to lock their screen we always prank them pretty hard. The greatest thing we ever did was in our latest release we had our managers buy us drinks at the pub. Well our managers left to go to the pub and one of them forgot to lock their computers. So we downloaded an image of the blue screen of death and made it full screen, unplugged his mouse and keyboard for added effect, and locked his computer. He came in the next Monday and couldn't log in because his keyboard wouldn't work. He actually called IT to bring him a new keyboard and mouse... then he unlocked his computer and freaked out with the IT guy because there was the blue screen of death. Needless to say he got a brand new computer as well as a new mouse and keyboard.

I'd call it a win win?