Revealing mistake: During the scene where the two teenage stars are outrunning the tidal wave through the hills, the girl is supposedly carrying her baby sibling strapped to her chest. However, the legs that can be seen dangling out of the baby-carrier belong to a dummy.
Revealing mistake: When the Interceptor team flies in their cargo plane to Mexico to investigate the UFO crash, several shots filmed from the front show the six of them all in the cockpit (not in the body of the plane.) They're seated as they might be in a car, three across, front and back. The window of the cockpit looks remarkably similar to a station wagon windshield, down to the large rectangular front windshield, and even the rear view mirror is in place.
Revealing mistake: As Reg fights with one of the baddies with the chainsaws, as the baddie take a swing with the chainsaw he misses and cuts into the wooden door. You can clearly see that a vertical cut has already been made in the wood though, to help the chainsaw get in and cut. (01:17:55)
Revealing mistake: In the scene where Will Smith first jumps aboard the rear of Loveless's train, as he climbs onto the roof of it you can see contrails from a jet aeroplane in the sky (top left).
Revealing mistake: After the bounty hunter, who changes three times throughout the movie, changes into the Reverend, he is in front of someone else who is a stand-in for the real Reverend but is obviously not.
Revealing mistake: When the little girl runs towards her grandmother and knocks another old woman out of the way, the other woman crashes into the wall of the terminal and it wobbles, showing it's a thin set wall. (00:14:35)
Revealing mistake: As Machine Gun Joe runs slowly runs over a fisherman in a small brook, he stops and "peels out" on his head. You can see a film crewman in the driver's seat doing the actual carnage.
Revealing mistake: This film was shot in Page Arizona with is pretty far north in the "high country". When they first drive up to the meteor side, there is Saguaro cactus that are only a couple of feet high with huge arms and a neon greenish color. These only grow on the desert floor in Arizona and they certainly are not neon colored. Additionally, the arms don't start appearing until they are much taller as they grow them to balance themselves since they have a very shallow root system.
Revealing mistake: When Tony is having his first fight in the embassy, it is obvious that most of his punches do not connect at all. The people he supposedly hit still react as though being hit, and the sound effects of a fist hitting a jaw are still there, even if Tony misses by several inches.
Revealing mistake: When Dana is walking down one hall with Leroy and a bottle of champagne, the exit sign is reversed. (00:53:05)
Revealing mistake: In the big speedboat scene near the end, when Cage is hanging off the side of the boat the stunt double is obvious in wide shots, where the entire boat is visible from above. He has big long sideburns, which are in stark contrast to Cage's short hair, visible in closer shots. At various other points in the movie, stunt doubles appear of much different physical builds than the two actors, with markedly different hair colours, and I seem to recall that at least one of them had facial hair.
Revealing mistake: Aeon's neck freckle swaps sides in one shot, revealing a flipped shot.
Revealing mistake: When Ellie is traveling through space, she unbuckles herself from the chair to retrieve the compass. The chair then breaks off from the violent movements of the space pod, and slams up against the ceiling. At this moment, Ellie is in deep space in a completely weightless environment. She is floating, suspended in the pod and the chair itself floats a bit before being slammed upwards by the force of the pod movement. But, some small parts of the chair (bolts, etc.) are seen falling directly down, towards us in obvious full gravity; revealing that the scene was actually shot in normal gravity.
Revealing mistake: When Will flies back in through the window and picks up Royal Pain, you can see that the back of Will's jacket is sticking up, which was where the harness was lifting him up. (01:24:05)
Revealing mistake: While Nick and his father are driving home with the giant baby behind (an obvious green screen projection), the baby breaks a tube from the car and throws it to the floor. Nick bends down to pick up a prop located offscreen, to add up for realism, but the problem is that he bends down before the baby throws it, so the whole trick is revealed.
Revealing mistake: When Dr. Railly is tied to the bed in the motel, her bonds are not very secure. In her struggles, she very nearly frees her right foot, by accident.
Revealing mistake: On Miranda there is a shuttle crashed into the side of a building, but there are no burn marks or a trail from where it had crash landed. The shuttle couldn't possibly have crashed into the building horizontally without crashing diagonally/vertically first, landing horizontally and then sliding into the building. Joss Whedon even states in the director's commentary that some burn marks would have been nice but it only occurred to him about 14 months after it was filmed. (01:17:15)
Revealing mistake: When the cop makes a mold of Dave's face from the impact site, there are nostrils in the mold, which are not possible to make with such an imprint.
Revealing mistake: During the scene where the military loses control of London, and their snipers are told "all targets are free", you can see heavy padding beneath Infected/civilian victims' shirts, etc, as they're shot. Presumably it's to stop them injuring themselves when they fall over, or to hold apparatus for the blood effects, but it's too bulky to be part of the characters' clothing.
Revealing mistake: Just before the end credits, as Superman is flying past Metropolis and into space, watch the car headlights in the background. The background shot is blatantly sped-up by a large amount, as all of the cars appear to be travelling at huge speeds, even in a traffic jam.