Worse than a brain fart, a brain shart has far worse consequences. A brain shart is doing something completely idiotic but in a key situation, or in a way that brings about serious repercussions.
Who can ever forget Chris Webber's brain shart in the 1993 NCAA championship game in which he called a timeout when his team had none, thereby getting a techinical foul and giving UNC the crown.
Because of a colossal brain shart on air in front of millions, Imus is out of a job and out on his can.
Similar to a brain fart, only something nonsensical comes out of your mouth during the thought process (aka a brain fart where a little comes out).
Guy 1(in deep thought): Yeep.
Guy 2: What?
Guy 1: Huh? Oh, sorry... I had a brain shart.