More than one plug
My favourite was being called over by an irate user with the classic "My computer doesn't work"! We know the next step, inquire if it was turned on and get abusive confirmation ending in "do you think I'm stupid?" (DO NOT ANSWER, really, just don't). Machine is whirring and lights are blinking, but no picture. Diffidently enquire if they have turned the thing that looks like a TV on at the mains. Relish sight of scarlet-faced user and confirm opinion (silently, obviously) that they are indeed a bear of little brain.