Boards
When your PC mousepad acts like a click and hold effect.
This only ever happens by accident, doesn't it? I've never been able to work out what subtle movement is required to make this happen if I need it to but I can surely end up dragging things over the screen and pulling tabs out of my browser when I don't want to.
Also I had to show a Mac user the two-finger scroll thing on their mousepad the other day. They'd never heard of it. I guess Apple just assume everyone knows...