A story I read once, no idea if it's true:
A mainframe at a university would shut down with no warning, usually a little after midnight, then a few minutes later power back up. Nobody could figure out why. Finally, some desperate grad students decided to sit and watch the computer in person and see what happens. And what they saw at the appointed hour was a janitor coming in and unplugging the power cord so he could plug in his vacuum cleaner.