I know a guy who once took a screen shot of someones desktop, flipped it around so it was upside down and set it as their wallpaper. He then removed their desktop shortcuts, hid the taskbar, flipped the screen so the picture he had set looked right, then inverted their mouse settings. Something like that anyway. Everything seemed perfectly normal except they couldn't click on anything, and they couldn't bring the task bar up since it was at the top of the screen now and they weren't looking there.