One time, the temperature probe accidentally fell out of a reef of mine and the chiller never knew to turn itself off. By the time I got home to notice, the tank temperature dropped extremely low, down to about 60. It felt ice cold though, and there was condensation on the front of the glass. I quickly put in about seven heaters cranked to their maximum. Good thing I saved all those heaters over all these years, within maybe 30-60 minutes I had the temp rising fast to it's normal range.
I lost some corals because of that and now always make sure all probes are extremely secure in place!

TPBM: What type of fish have you had that you will never want to keep again?