Had a few incidents over the years. Some were equipment failures (GFCI trip in one case, lost a pair of flame angels in the affected tank - they were the only fish in there), and some were livestock reasons and not necessarily "tank crash" but still fit the posted definition (had an extremely large H. magnifica anemone for many years that was capable of killing off an entire tank of fish if it so much as burped. In fact, the reason I sold it after close to a decade of caring for it was that it was just too lethal for my tastes
).