Throw artist

A throw artist is a person who has "thrown a game," or lost when they were almost guaranteed to win due to making an egregious error.

The phrase can be used at any point when someone throws a game, but is more frequently used to refer to someone who has thrown games multiple times in succession.

My friend: *Loses a game after being 30 points ahead*
Me: Damn, we've got a professional throw artist over here!