True story that I witnessed myself (Uh... Did I say witnessed? I mean caused :) : A couple of years ago, I was getting bored in computer class, and fired up MS Paint. Not knowing what to draw, I opened the Clouds default windows background, and started playing around. Finally, I had a revalation: An error message! I'd been trying to learn how to draw in 3d, and a window would be good basic practice, and I might as well make it funny.
To make a long story short, I got the next period computer teacher really good. She was a Mac user, and had no idea what a Windows error message looked like. She came in the room, took one look at the screen, and class was instantly put on hold to figure out what was going on. It ended up taking 15 minutes and interrupting ANOTHER teachers class before anyone managed to figure out what had happened - My nice little picture had been made the windows background, resulting in a somewhat realistic and very unclickable error message on the screen.