I didn't walk out of the Avatar movie, but most people did. Halfway through someone opened the door to the theater and yelled in, "avatar sucks!" Normally I'd be a bit put out by such a disturbance, but in this case it was actually the most enjoyable and funniest part of the movie. (Last Airbender just in case)
I imagine they mean M. Night’s 2010 movie, The Last Airbender. I thought this was pretty obvious, but upon Googling learned that his film was not called Avatar: The Last Airbender like the show is.