Revealing mistake: During the fight montage, the winner of the first match wins by kicking his opponent in the face. His foot never makes contact.
Revealing mistake: After Johnny Five falls out the window and whips out his hang-glider, as he turns right-side up and soars past the buildings, you can see that his body is transparent (an artifact of the optical matting process used).
Revealing mistake: The palm trees next to the building Arthur came out were not real trees. One is flush against the front wall of the building and another is in a nearby corner. The palm tree in front looked completely flat - like a large piece of cardboard or maybe painted on the wall. Palm trees should be at least 30' away from a structure as well as outdoor pipes, buried lines, etc. They grow a big central root below the trunk and MANY thin roots running horizontal close to the surface - some may be exposed. (00:05:51)
Revealing mistake: When Dolph goes into the bar near the beginning, he headbutts a bald guy. You can see that he doesn't even come close to making contact. Still, the guy has a broken nose.
Revealing mistake: In the bathroom scene when Debbie is attacked by the clowns from the medicine cabinet, you can see the holes where the puppets are being controlled from.
Revealing mistake: Clearly not Susan Blakely when she goes topless. Her hair suddenly becomes curlier, that's a body double.
Revealing mistake: When Jemma enters the headshrinker's laboratory, there is a table with three heads on it each encased in glass. When it goes in close-up on the middle head, you can see that the table has a hole where the actor can slip in his head.