Finally, someone sat up all night to watch what happens and observed a cleaning lady unplugging the machine from power in order to plug in a vacuum. When she was finished, she plugged the computer back in and left.
Another on of my favorites, not bug but more malicious, was someone wrote a program to swing the heads on one of those old giant hard drives back and forth. At the right frequency, the back and forth momentum would cause the drive to walk away from the wall and eventually unplug itself.