I had some hands-on computer repair training at a private school once. One old machine wouldn't boot, complaining that it couldn't find the keyboard which was plugged into it. I unplugged it while the computer was on. At the time, unplugging a keyboard while the computer was on was... not a good thing. There was a little curl of smoke, a scorch mark on the motherboard, and a sustained tone from the chassis and that computer breathed its last.
Later, in college, I used the "net send" command on random people in open labs just to watch how confused they got.