I had a goldfish die due to a jaw deformity that kept it from eating. Had to euthanize; the poor thing got progressively more emaciated no matter how small we ground the food for it, and it eventually developed dropsy.
I've had numerous bettas die, probably due to the fact that I buy sickies for rehabilitation purpose, and some just never fully recover and thus die young. I've also had a few randomly die for no apparent reason (possibly a bad heart, or age??).
Then, of course, there is my vicious little goldfish Tangaloor Firefins, who killed his first two tank mates (both goldfish). I stopped giving him "friends" until he was larger, Grizraz (my other goldfish) was much larger, and I had a big enough tank for them to share. They still get into little tiffs, but now that he is in with a larger fish, he never wins.