Britney Spears Walks Off 'X Factor'

Britney Spears walked off the set of "X Factor" moments ago ... and according to several people in the audience, she was upset after a contestant butchered her song, "Hold It Against Me."

Britney did not come back immediately ... and four contestants auditioned in front of the remaining judges ... and an empty seat.

Someone connected with the show tells us Britney was not upset over the song -- and that she "Just took a short break."