Revealing mistake: Just after Mr. Creosote explodes and all of the guests are vomiting, there is one male guest third from the front that is puking. Though his hand is up near his mouth, it is clear that the puke actually comes from his hand rather than his mouth, revealing that he is holding something that produces the fake vomit.
Revealing mistake: During the sperm song in the "Miracle of Birth part II" there is one shot where the kids are dancing down the street. Watch the girl at the front on the left. She hesitates and then runs to catch up to the front line. Then when they go round the corner, she has to be moved along by the other kids as she stops to stare at the camera.