Many, many years ago, rather than individual PCs, everyone used "dumb" text terminals attached directly to a central server. Every keyboard press went to the server, then the server would send something back to the terminal, e.g. to put a letter on the screen or perform some other action. This meant the central server had complete control of the terminal. Which meant, of course, that if you had complete control of the central server...
I wrote a little program that would read the contents of the screen, then start to make random characters "fall" to the bottom of the screen, knocking into other characters on the way down and making them fall as well, in a kind of cascading waterfall effect. This got faster and faster until all the characters would end up in a big random pile at the bottom of the screen. Then, after a few seconds, if the person hit a key, the screen would magically "jump" back in place, as if nothing had happened.
I used to run this occasionally on the terminals of our new employees, and when they'd ask in surprise "what the hell just happened to my screen?!?!", I'd say "oh yeah, some of our terminals are a bit old and the characters sometimes lose hold of the screen and fall down, don't worry about it...". Some people put up with it for weeks before asking for a newer terminal. Even the technical people who should have known better would often just accept the explanation and walk away, then a bit later (sometimes days!) would come back with "hold on!...".
It still cracks me up when I think of it! :-)