Goldfish are greedy and opportunistic omnivores- if they can catch it, and it fits in their mouth, then they will eat it (I've had to tweeze out a few pebbles in my time). They are suckers, literally, and do not have teeth. So, if they were to eat another goldfish, they'd pretty much have to keep attacking and attacking until the other fish is weakened unto the point of near death, or death itself- which would take a lot of time, unless the other fish is already sick, because no fish just hovers and takes the hit after hit.
You'd pretty much have to be a very neglectful owner not to notice the attack to begin with, and even more so not to realise there is an upside carcass bobbling around the tank.
So I would say it was the time you realised just what you are and how we all are; because you watched what was happening, and did nothing about it.