I have two:
1. I had a black ghost knife get caught by a fidler crab (this was a long time ago, pre-internet, and they were "new" to the hobby... oh, and I was pretty stupid). The crab basically caught it with its large claw, and proceeded to eat it alive. By the time I got home, the knifefish was dead.
2. More recently, I had a colony of multies. I had not paid attention to the tank for a couple of days, so no clue how long it was in this state... but one day, I was standing near it, and I felt all of this heat coming from somewhere. It was dark, so I could not see in the tank right then. I touched the glass, and it was HOT. I was like, whoa! I turned on the light, and all fish were dead. The heater... a Stealth heater... had malfunctioned, and cooked the fish.
-Rich