Years ago, I was a fiend for Africans. Had a 110 set up with a collection of haps that I had grown from babies. Breeding groups of moorii, compressiceps (I know they're not haps anymore, but there were then), a few other odds and ends and a trio of gorgeous peacocks. Plus a few big balas as dithers. Note the sizes of those fish and the size of the tank, it was heavily stocked, but heavily filtered.
Went on my usual summer backpacking trip to the Sierras to regain my sanity, leaving my mom to take care of the tank. Came back after a week to find out that the filter had failed, and despite the best efforts of a coworker from the LFS I worked at, the tank crashed. All dead. Heartbreak. I couldn't even be mad, because that's the risk you take when you ask someone to watch your fish. On the positive side, it became my first SW tank.
The most unpleasant event happened when I lived inTucson. Went away to teach for a few weeks in the summer, and had my idiotic neighbor look in on my tank. It was a 29 with a breeding pair of angels, plus a small school of mature cherry barbs. At some point, the filter failed, and the cooler in the house went off. All dead. But the worst thing was that the little creep didn't even look in on the tank to remove the bodies. Imagine walking into the house, smelling the rotting soup of week old dead fish cooked in the Arizona summer. I almost hurled.
Now, every tank has backup circulation, and is stocked so that it can last a few days without power. The outages this summer tested the systems, and they came through fine. I'm still a little concerned about next week, when we both will be out of the country, but I have a reefer across the street I can trust to check things.