Revealing mistake: During the close-up of the gas canisters, the condensation is running upwards and "into" the valve. (01:31:10)
Revealing mistake: You can see the stick making the finger move when it's in the box. (00:38:20)
Revealing mistake: When the demonstration crowd is assembling in the plaza, there's a yellow tram entering on the left hand side. Watch as someone walks across the tracks in front of it and disappears through the corner of the cab. The shot must be a composite of the crowd amassing and the tram shot separately and then overlayed.
Revealing mistake: When they get Renn back into the room after he has been shot in the face with something acid-like, the smoke comes from behind his head. Not his face.
Revealing mistake: When Kristen is running through the hallway with the girl in her arms you see that it is a doll. (00:07:00)
Revealing mistake: Many times throughout the movie, people are shot and they bleed, but their clothes don't have holes from where the bullets hit them. The most noticeable examples are when the teacher is shot at the end, and when the big gun fight between the girls breaks out in the lighthouse.
Revealing mistake: In the scenes where Riley is using his computer to look up information on the "SearchInfo" site, the screenshot used is of Wikipedia. (You can even see the "About Wikipedia" link on the left).
Revealing mistake: When the kid and the coach are throwing the little boxes of corn flakes in the air the kid's last couple of boxes were pre opened.
Revealing mistake: To save money during the beach picnic scene, Welles used stock footage from King Kong (1933) for the jungle background. You can quite clearly see pterodactyls flying in the background (confirmed on DVD commentary).
Revealing mistake: Towards the end when Harmony calls Perry's phone as she's lying on the ground, Harry grabs the phone and looks at the caller ID to see her name. Unfortunately that's not the display showing who's calling. Anyone who has a Nokia phone will recognise that that's showing the contact list with just two entries - Harmony and a random number. The fact that it says "contacts" at the top of the screen is a bit of a giveaway.
Revealing mistake: In the barber shop scene at the end of the film, Agent Anderson terrorises Deputy Sheriff Pell (who has a full head of hair) with a razor blade. Anderson throws him across the room, but the person being thrown across the room has a mostly bald head.
Revealing mistake: When Stevie's car is hit by the delivery truck, the side shot that shows the car beginning to roll reveals there is no engine in the car, as we can see through the front fender wells to the truck as it passes on the far side of the car. (01:15:40)
Revealing mistake: During Bartleby's "Prepare to taste God's wrath" diatribe, you can clearly see the seam along the front of his shirt that he uses to tear his shirt open revealing his armour. This seam is not seen at any other point in the movie.
Revealing mistake: When Jesse is cleaning out his room, he puts a tape into his tape player, and presses a button down on the top of it. But halfway though the scene of him packing his things away, you can clearly see the button is no longer pressed down, yet the tape still plays. (00:18:10 - 00:19:00)
Revealing mistake: When Gretchen turns around while on the bed, a shard of glass in the frame flaps in the wind.
Revealing mistake: When the musicians stop playing and the world leaders start getting up from the table, it goes to a shot spanning the entire table, but also showing the musician's stage. This is where you see the mistake, as you see Berkhamp suddenly at the front of the stage wearing his headband and dancing, when it cuts away from that shot, it shows Berkhamp is still sitting behind his instrument with his headband still off. Then we find that out of place shot was recycled from when Berkhamp actually was onstage and dancing as we see the exact same shot again less than a minute later, this time the shot is where it's supposed to be however this is still a mistake and quite an odd one. (01:26:25)
Revealing mistake: During the scene in which Lou forces himself on Kim in the lunchroom, donning a black ski mask, Kim's brother Alex jumps up to his sister's defense. Just before Alex throws his first punch, Kim screams "Michael," referring to the actor's real name Michael Tough. Kim then correctly screams "Alex," the name of the character he was playing in the film.
Revealing mistake: When Jason climbs onto the rail of the fire escape, it is not attached to the wall - intentionally, it has not broken - but then in another shot, two seconds later, it 'breaks'. It pulls out of a visible hole in the wall, although it was never actually attached and there was no hole before. (00:23:25)
Revealing mistake: As Mary Ann is dying, Kevin is holding her close to him, hugging her. From the behind shot of Mary Ann, you can see, sticking out from behind the right side of her neck/hair, is a tube and the yellow packet that held the blood/pump. (01:53:00)
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.