Most of mine have already been said. does that still count?
....I...Glass heater+only partially covered with water + splashing water from a filter = broken heater and electrocuted fish....
1: did that... 3 times... ok only once was from a water change. the other two (a week apart) were from using glass heaters in my dump filter (do we have a headslap emote?). now I have the unbreakable, auto-turn-off-if-its-not-wet kind in there.
....I have a habit of overfilling tanks. ...
2: I have done that twice in the past year on my 75. The last time was just two weeks ago. But it was only a couple of gallons. My husband still asked why the floor was squishy :unsure:
.......2. Not covering up breeding vats/ponds from herons with bird net. I lost 2 colonies of Buccochromis Rhodesii, 1 colony of Champs Careleus, 1 colony Stigmatochromis Modestus, 2 colonies of AER's, 1 colony of Ngaras, and a mixture of stuff to grow out some of which were going to be used for show

....
3: Way back when I lived with my parents I put some fish outside in a tub. I did cover it. but racoons are smart little creatures... and good at fishing
....I drained the tank, cleaned it some, filled it and put the fish in. Went about changing the water in some other tanks. Looked back about 30 minutes later and 100 dead AER's.
Forgot to treat the water....
4: I did that one too. Got distracted and didn't add the dechlor. a little while later I look to see a bunch of fish looking really freaked out and swimming funny (if they were swimming at all).

only about a dozen lost though, not quite as bad as yours though....
oh, I do have one that hasn't been mentioned yet...
5: Again back when I lived at my parents house they had a love seat that was kind of a room divider between the living room and the kitchen. It was the kind that has the fold out foot rests. I decided to add to the room divideing effect and put a 29 gallon tanginikan tank on a metal stand behind the love seat. One day one of us (I really don't remember who, I useually sat on the couch) put out the foot rest without realizing the loveseat had been bumped back a bit. The seat leaned back and knocked the whole tank all over the kitchen! there were fish swimming amongst broken glass and shoes. I tried to save them. But that was a bit much I guess, especilly since I didn't really have any extra tanks and they ended up in mixing bowels and stuff......