I once found one of my favorite fish dead after it choked to death on an earthworm twice as long as he was. It was a mudminnow, and quite the personality, so I guess it was a fitting way for it to go.
The most sick I've ever been over a mistake was with my loach tank. I had a bunch of loaches, and about a third had wasting disease because they had been wild caught, so I wanted to worm the tank, but most worming medcines aren't super effective against WD. So I did some research and I bought this wormer that was supposed to work well, but it was sold as pig wormer, and thus very difficult to measure (it was supposed to be mixed with the food). I decided to try it first on a loach that was pretty far gone to make sure it was safe. After about 20min it was fine, and I had to leave to go somewhere so I gave it to the rest of the tank (botia - striata, dario, historanica, kubotai, and sidthumunki, rasboras, gourami, etc.) When I got back everyone was dying. And it was a horrible, long drawn out process. I literally could do nothing as all these rare loaches I had tracked down slowly died. I was just sick, and wished so much I had waited longer before giving it to them. Never do anything other than really basic care when you are in a hurry. I think only 4 fish out of about 50 lived.