I worked in a lab with a lot of inherited energized superconducting magnets, operating in a persistent state (think of MRI magnets, but smaller). We had to move one of them, which required powering it down. Powering down a magnet like this requires special steps to gradually reduce current. We knew what the current was in the magnet, but not the polarity because no one had bothered to record it. So, flipped a coin to guess. Guessed wrong and quenched the magnet, huge plume of helium vapor, probably a few thousands of dollars worth. Always record your currents and polarities!