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 Douglas' house which falls off the cliff at the end of the film was built to do just that. It was only used for exterior shots, and the interiors were filmed on a sound stage. If you watch carefully as the house crashes, you can see that it has no interior.
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: When the tank runs over a car during the riot scene, you can see that the car has no engine.
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.
Revealing mistake: When the swimmer is lifted by the periscope she is wearing skin coloured underwear around her thighs, despite she is supposed to be naked.
