If they use a Windows computer, type the command Ctrl+Alt+DownArrow. It turns their screen upside-down. The other arrows work as well to turn the screen sideways. Not sure how familiar most people are with this though. I've always wanted to try it on someone and see how they reacted.
It might be a Dell thing. I've only tried it on Dells and I've seen it work on a couple laptops and an older desktop. Haven't really tested this extensively though. YMMV.
