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 Al is running from the fire in the incineration chamber, fire is shooting out the back. When the fire goes away lights can still be seen in the chamber entrance. The fire should have incinerated them, because this area would be sealed off from the hot area during normal operation.
Revealing mistake: During the shoot out with McGivens's gang at the Cortez ranch, the wife of Cortez sneaks out of the trap door with her baby and peers around the side of the building. You can notice that the baby is really a doll.
Revealing mistake: There is one shot of the Duke boys in their car talking to Uncle Jessie with the CB radio. You can hear Uncle Jessie respond (through the radio) while Bo/Luke still has the button on the microphone pressed. When the button on the mic is pressed, no sound comes out of a CB radio at all, as they cannot transmit and receive at the same time.
Revealing mistake: When the talons and E.D.I. attack the fort during their first mission, the explosions show the use of pyrotechnics when A.T.S. missiles explode in a simple blast.
Revealing mistake: When the Mob's FBI is about to storm the house they shoot the transformer cutting power to the house. But yet when Bruce Willis shoots the fountain in the house it is still pumping out water.
Revealing mistake: In the battle at the cathedral and at the end of the film when the footage is repeated, we see people hacking away at a torn up, dead body. The problem is their swords and axes are not damaging the body at all, revealing everything is a prop of some sort.
Revealing mistake: Right after Darius blows the boats, Steele is on the dock firing at him. It is shown that he shoots a couple rounds but his finger is not on the trigger. (00:24:00)
Revealing mistake: When Ryer and Rand jump off Rand's window and down to the street, they land on a tree. Notice the severed branches; instead of being broken [presumably from the falling bodies], they are cut rather smoothly.
Revealing mistake: When Chas is being thrown up and down between the ceiling and the floor, if you look closely as Chas is falling from the ceiling to the floor and goes up again, when he goes up it's just a rewind of him falling down. (01:28:35)
Revealing mistake: During the car chase with the railway train wheels, when some of the cars get destroyed and flipped, you can see flame and smoke from the bottoms of the flipped cars where the cannons fired to flip them.
Revealing mistake: In the scene when the hijacked bus gets crashed, you can see the tire skid marks on the road already from where they have performed multiple takes of this stunt.
Revealing mistake: When Victor Von Doom is shown proposing to Sue Storm in the space station, for a short moment you can see a reflection of the blue screens they used to create "space".
Revealing mistake: Visible blood pack on the waitress's back when she gets stabbed by Otis.
Revealing mistake: When Charlie is climbing the wall you can see a handhold screwed into the wall when she lets go with her right hand. (00:59:45)
Revealing mistake: Quite obviously Steven Seagal is not jumping out of the warehouse window himself, it's blatantly a stuntman.
Revealing mistake: When the guys are on the jet skis, when they are showing off, you can tell they are stunt doubles.
Revealing mistake: After the car chase, the authorities are seen carrying away two body bags that are clearly empty. The men show no strain at all in carrying the bodies, and the bags even look empty. But if they were, they'd just be folded up and taken by one person.
Revealing mistake: When Walter, Danny and the astronaut are in the kitchen breaking stuff to burn, the smaller boy tries to smash an old style broom on the table. He tries three times, on his final try the shank breaks in half, revealing where it had been sawn nearly all the way through in nice clean line, with only a splintery piece showing where it really snapped.
Revealing mistake: During the fight in the apartment, the girl in the shower is wearing a black thong. (01:44:00)