Revealing mistake: In the scene at the end of the movie where Lee Marvin is running interference during the escape by shooting in the air, horses pulling a beer wagon get startled and run into the street where the wagon turns over. At the beginning of the shot, when the horses begin to bolt, you can see the stuntman driving the wagon. He is sitting low behind the wooden front of the wagon (which has holes cut in it so he can see out).
Revealing mistake: When The Deputy Charley Rakes goes to Cricket's house and attacks Jack, you see Charley throw Jack down the steps outside the house. If you look closely, you can see he lands on something really soft and spongy like a mattress which is hidden under the foliage on the ground to break his fall. When Charley walks down the steps towards Jack, you see him walk on it, which again looks really soft and spongy.
Revealing mistake: The caboose and two bunk cars are uncoupled and sent rolling down the hill, with troops locked inside. As the cars roll off a cliff, troops can be heard yelling. However, as the cars hit the ground and shatter into rubble, the wood does not appear to be blood stained, nor does there appear to be any sign of bodies in the debris. The cars were obviously empty for the scene, and given the circumstances of the soldiers allegedly trapped within them, it's unlikely that any of the soldiers could've gotten out in time.
Revealing mistake: When we see a wagon with a woman and several children in it, the horses get spooked and run. A woman is seen desperately trying to stop the runaway horses. In the next scene you can see the children are not real and the woman is a man dressed in a woman's dress, obviously a stunt man used for that scene.
Revealing mistake: In the scene where the boy is shot in the leg and the carried away by an unionist soldier you can see more union soldiers storm past them. The bayonets of the soldiers are obviously made of rubber as they are dangling while the rifles are moved up and down by the running soldiers.
Revealing mistake: After the trailer explodes on the bridge, towards the end the truck swerves to the left. As it drives off the bridge there is no tail shaft between the gearbox and front diff - the truck falling off the bridge is just a shell.
Revealing mistake: The gang's visit to Northfield, Minnesota reveals a typical late 18th century town, with dozens of people on many horses and horse drawn wagons. All the horses leave deep tracks in the dirt, because they are walking on freshly laid soil that has not had time to compact. There is absolutely no horse manure to be seen anywhere, despite dozens of horses. Clearly, the film makers used a 20th century town and laid new soil over the pavement shortly before the filming began.
Revealing mistake: The outline of Divine's wig is obvious during the closeup when she's scolding her donkey at the beginning.
Revealing mistake: When Jordan and Jett go into the liquor storage room for Jordan to teach Jett a lesson for beating up Jordie, Jordan instead throws a large wine basket at the shelves of wine/alcohol. The shelves start to fall before the basket even hits them. (02:44:55)
Revealing mistake: When Caleb shows Mae his horse and it rears you can see the man holding the horse's rope is not Caleb. The man has an older, redder face and greyed sideburns. (00:08:21)
Revealing mistake: When the protagonist jumps two of the evil cowboys that are after his jewels, the stuntman on the right tosses aside the rifle in a quick deliberate move pretending to lose it. (00:10:25)
Revealing mistake: During "Many a New Day", when Laurie sings "many a red sun will set" and before they all hear the Cummings girl laugh outside, Laurie and two others are looking into a mirror. The actresses are actually looking into the camera through a mirror frame, to give the illusion that we are seeing their reflections. When the girl with the long black hair joins them, you see her run in from the side, then her "reflection" in the mirror. However, if it was a real mirror, you wouldn't see her run in from the side. (00:44:00)
Revealing mistake: During the bison shooting scene, a bison goes down close to the camera and both horns jiggle like they're about to fall off. Looks quite fake.
Revealing mistake: At the end after the driver of the wagon has fallen off and the horses are running wild you can see a slot in the front of the seat and arms holding the reins.
Revealing mistake: When aircraft 5734 taxis to the runway for its first take off and comes to a stop in front of the camera for the takeoff checklist, it's clear that it is just a photo shoot and not a real take off because the jet engines still have their intake covers on. Same thing after it lands and is taxiing back to its parking location where June Allyson is waiting for Jimmy Stewart to return. The jet engine intake covers are still in place.
Revealing mistake: When Potter starts shaving at the morning of the Indian attack on the wagon train, his mirror falls off the post, and as he bends down to retrieve it, an arrow embeds itself right above his head. If you look closely in that scene, you can see a length of targeting wire used to guide the arrow sticking to the post above Potter's head. (00:37:10)
Revealing mistake: In each one of his scenes, every time you see Captain Fuller you can see that the actor is wearing contacts to make his eyes blue. It's obvious contacts with the brown rings of his natural eyes around them. And contacts were not invented for a long long time after this movie takes place.
Revealing mistake: In the Gatling Gun scene when the camera cuts to a short couple frame shot showing a closeup on the .45-70 Government ammunition casings on the ground that are falling from the gun, the casings are star crimped. Star crimped casings indicate blank ammunition. Standard ammunition isn't possible with star crimping. The size and casing shape indicates .45-70 Gov which was popular in these guns at this time. (01:46:40)
Revealing mistake: The Giles brother who got "hung" when he fell after getting shot by MacMasters can be seen moving his body (mostly arms). (00:04:06)
Revealing mistake: Many of the cavalry soldiers were carrying M-1 Garand rifles (WW2 era rifles).