Derived from the common Microsoft computer error, User Error is called when someone screws up. This could be knocking over a glass, saying a stupid phrase, telling a bad story, cheating on your girlfriend, forgetting to feed your child, etc. etc.
John (to hot co-worker): "Hey Marissa, apart from being sexy, what do you do for a living?"
Marissa: "User Error"
Stephen walks into the room carrying a plate full of appetizers for the party, but trips and spills them on his mother-in-law. User Error