Revealing mistake: After Steelleg's and Iron Head's song, Steelleg is taken outside for a fight with some thugs. He kicks a ball into the stomach of a guy, making him fly backwards and roll over several times before he slides away on the ground. Look at his neck in the shot from behind - you can see the construction he was dragged by under the shirt.
Revealing mistake: When the Kung Fu team is playing against a football team (before the team that has two women), one of their players is running with the ball stuck to his stomach. He knocks two players down by running into them, and the goalkeeper jumps on him, holding him by his clothes, but he is dragged after the player. You can see him float above the ground for a moment (while holding the player's clothes), just before the goal is made.
Revealing mistake: When the six brothers are reunited at the rooftop of the building, we see the fat boy walking with a scarf around his neck that is waving in the wind. If you look closely, you will see the string that makes this effect possible.