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: They wouldn't be able to walk or run nearly so easily when near a tornado. In ametuer video footage of real tornados, the people could only move quite slowly, and were walking on an angle of 30 degrees from upright.
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: 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: 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: It's very clear they blew up a model of the buildings. The grass seen just before the explosions is way too long and is not as long when the characters are running away. Further, the truck and the barrels have disappeared at the explosion shot.
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.