Revealing mistake: When the Ferris Wheel at the theme park has broken from its mounting and is rolling down the pier, it appears to be rolling forward but rotating anti-clockwise when if it is rolling forward it should be rotating clockwise.
Revealing mistake: The actor who played "Wally" did all of his own dancing during the big dance scene, except the part where he tap-dances down the long table of food. We see Wally jump up onto one end of the table, then the shot cuts to a close-up of his feet as he tap dances among the plates and glasses. When he reaches the end, he jumps up into the air and drops into frame, showing that it's really him. Since the actor couldn't tap dance, the film's choreographer did the tap dancing part. At the end of the table they hung a trapeze bar and the actor hung from it with his feet pulled up. When the choreographer reached the end of the table, he jumped up into the air and grabbed the bar just as the actor let go, so it looks like the same person. But if you look very carefully, you can see the actor's shoes dangling above the edge of the table just before the choreographer jumps.
Revealing mistake: During the riot scene, "Stretch" drags Betty under a truck so he can rape her. Wally drives the tank across the street and bumps the truck, knocking it over (so he can save Betty). If you watch, the tank bumps the truck, there's a pause, and then the truck flips over. It is obviously pulled over by something off-screen and not by the tank.