It's not uncommon from what I have read from other people, but it's not common either. I know that sounds contradictory, but it's something that no one expects but you shouldn't be surprised when you see it. It's like when a hard drive in a computer takes a dive. It's completely normaly, but we're all shocked when it happens.
What are your ammonia, nitrite, and nitrate levels? Was this fish in a breeder or the main tank? Fish die of stress and shock a lot of times. What else is going on in the tank? Maybe we can narrow some things down to prevent this from happening again if it's the environment that is causing it.