Revealing mistake: When Sweet Chuck is sitting on a bench in the gym reading a paper, the big heavy comes down and sits next to him. Sweet Chuck goes flying up in the air a second before the big guy sits down.
Continuity mistake: At the end of the movie, the two balloons are in the air. As one of them goes down to land, we see a shot with balloons on the ground. In the next shot, one of them is still up and flying.