Closer to the end of this video when all the Boys and the dancers are dancing in that big ballroom keep your eye on the left side of the screen, where Brian is dancing. Keep watching him. If you look closely you can see a couple times he is behind on the moves then at one point when everyone gets down to the floor to do a move he gets down then just sits on the floor for a few seconds looking confused. The scene changes then goes back to him and he is back up dancing.