Revealing mistake: When Mason is having his hair cut, after finishing up, he is talking to Womack and turns his back on him. If you look at the back of Mason's neck, it is clear that he's not just been freshly shorn; there are whorls of hair just above his collar. (Either that, or the barber was terrible at his job).
Revealing mistake: When Stan falls into the dug-out at the baseball game you can clearly see the crash mat that he lands on.
Revealing mistake: In the middle of the movie, Roy is flossing his teeth by himself. In the mirror as well as when he turns around, you can see his hand holding the hook, if you look closely. (00:58:33)
Revealing mistake: Watch the scene where they're headbanging and the tape keeps going out. You can see where they splice the scene in because the trees change in the background instantly.
Revealing mistake: While driving the red truck through the rolling house, Dorothy is mounted on the back of the truck. There are some very sensitive and fragile instruments located on the top sides of Dorothy that stick up above the cab of the truck. These would certainly have been torn off while crashing through the walls of the house, turning Dorothy into a useless pile of junk.
Revealing mistake: When Barb is in the bubble bath, you can see the strap from a G-string she is wearing.
Revealing mistake: When WD-40 is parachuting onto the island at the beginning, he throws a match through his parachute. As the shot changes, you can clearly see the smoke canister that was producing the effect of the parachute being on fire, swinging from beneath the canopy.
Revealing mistake: After the Martian who is dressed up as a women spits a finger in the fish tank a bit of the Martian's mask near its mouth is torn away. As she chews the nitrogen gum the human lips move but her real teeth don't. (01:04:20)
Revealing mistake: Hamlet is killed and in the next scene is being carried with his arms stiff to his sides. The men carrying him go over a bump, jolting Hamlet, and he blinks.
Revealing mistake: When Arnie jumps from the plane, notice the stunt double. He weighs about half of what Arnie would, and has no muscles. (00:55:10)