Revealing mistake: In the scene where Maverick is 'taken away' to get his hands chopped off by his Indian friends, there is a vehicle driving down a highway in the distance. It can be seen in the far background, but it is obviously an automobile.
Revealing mistake: In "Wonderful, Wonderful Day" there are live birds flying on the set, but they were confused by the backdrop and several crash into it. (The director acknowledges this in a documentary.)
Revealing mistake: When Crockett, Bowie and their men are about to destroy the giant cannon, several of the men are standing right next to it at the moment one of them tosses the torch on the touch hole. Then, there's a big flash at the touchhole. In the very next shot, right before the cannon explodes, we see the cannon, but everyone has disappeared. There was no time for them to flee completely out of sight.
Revealing mistake: After the Indians have attacked the wagon train Robert Taylor returns to see the devastation. When they call out the names of the dead we see a close up of each person and some of the "dead" are breathing.
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: During the Little Bighorn massacre one of Custer's lieutenants turns to address Custer and is struck in the back with an arrow, and the thick pad or board is visible under the actor's shirt.
Revealing mistake: When the villagers of Santa Poco are making the Amigo outfits, the sewing machines are not threaded because no thread is seen in the cloth as it passes through the sewing machine needles.
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: When Mongo wakes up and breaks free of the chains, in the shot where he breaks free, the chains suddenly shift position and some even disappear. This is because the camera was stopped and the chains were replaced with fake break always before the camera was restarted. Another way to see this is because the water that dripping down Mongo's face suddenly stops. (01:02:15)
Revealing mistake: In the scene where the soldier has to have his leg amputated, when a soldier steps over him, he accidentally hits the knife, which bends, revealing it's a rubber prop.
Revealing mistake: At the end of the movie, when the last "Graboid" appears, in one or two shots of him retracting back into the ground, you can tell the shots are reversed, as dirt is being sucked down in an unnatural way. It flies up and sort of "implodes" towards the creature. (01:23:50)
Revealing mistake: During Cole and Mississippi's night patrol, a bunch of horsemen start making a fuss in the street. When Mississippi dives under their horses, there is an interruption between when he dives, and when he (supposedly) lies in the horses' path. Evidently, this scene was made by joining two cuts.
Revealing mistake: When Marsten's men are forcing Aborigines off a cliff, Quigley shoots one man who falls over the edge with his horse. Just as the horse and rider fall over, you can briefly see the rigging used to flip the mechanical horse over the edge.
Revealing mistake: When El Mariachi is trying to escape Moco's men by sliding down a wire onto the bus, he first throws the ball-and-chain-weapon over the wire. But then when he slides down, you can see a pulley device that he is using, not the chain. (00:26:45)
Revealing mistake: The bullet hole gouge in the window frame near Ricky Nelson and the outward facing splinters at the bullet hole in the door frame near John Wayne indicate exit holes.
Revealing mistake: During John Elder's fight in Hasting's gun store, he ducks behind a counter, and as he pokes out his head, Hastings fires at him. On the counter, the "bullet hole" - containing the powder charge for imitating a bullet impact - is already visible before Hastings fires his gun.
Revealing mistake: In the opening sequence when Sundance shoots the gun belt off the card player, the film was cut to make the quick draw appear faster. You can see Butch's image jump across the screen in the background.
Revealing mistake: When Tristan kill the guy in the liquor storage and the hay fork goes through his chest, you can see that parts of the hay fork's tines closest to his body are not covered with blood. (01:51:05)
Revealing mistake: Whilst Spencer Tracy is driving the Jeep through the desert at high speed, during one of the long shots the rough terrain causes the vehicle to bounce violently; if you look carefully you can just make out the stunt-driver's "missing" arm coming out from beneath his jacket to help wrestle with the steering wheel.
Revealing mistake: When the wheel breaks and the coach crashes, it is visibly empty. The five passengers who were riding inside have disappeared. (00:12:15)