Might as well post mine.
Years ago, I was doing a tank move and fished out all my fish to a 5 gallon pail. As the move was taking longer than expected, after some time I figured that the water must have cooled off too much, so I decided to place a heater to warm up the water a bit.
Even though I had the thermostat set to turn off when the water would heat up to the desired temperature, I "boiled" all my fish as I did not have a small circulating pump in the bucket.
It still pains me to think about those poor fish that day, even though it happened so many years ago
