The guns used in the movie are Colt single action revolvers model 1873, Henry lever action rifle look-alikes model 1860, and Winchester lever action rifles model 1892. The only correct period guns used were the single-shot percussion-cap dueling pistols used in the opening scene.
Major Henry mentions that Ed McBain spent 5 years at the Yuma Territorial Prison. In 1843 Arizona was part of Mexico, so there was no Yuma Territorial Prison then. The Mexican-American war, where the US took the SW from Mexico, wasn't won until 1848.
When McBain is interrogated Major Henry states that McBain's guns had been stolen from Fort Sill and that McBain had served five years in Yuma Territorial Prison. As the film is supposedly set before 1848, neither is possible. Yuma Territorial Prison was opened in 1876, while Fort Sill was first established in 1869. Both occurred after Texas lost its independence and became a state.
When Pa Schofield goes out of the house with the baby in his arms and calls his friends, the baby is completely covered. So we see the Cutter's hand discovering the baby's face in close-up. Next shot the baby is still completely covered.
As Jake and Regret are delivering their wagon-load of guns, they see see Indians on the hilltops. Jake removes a staff with feathers from his side of the wagon, and hands it to Regret, saying 'We'll see if this passport works.' The next two quick camera cuts show the staff first back on the wagon, then again in Regret's hands.
In the bar scene: Tully Crow (Lee Marvin) asks the bartender for a bottle of "the good stuff." The bartender holds up a bottle of whiskey with his right hand. Crow, not believing that it really is "the good stuff," gets mad and shoots the bottle and breaks it. The camera view then changes to a stuffed deer head on the wall, which Crow also shoots. Then the camera goes back to the bartender, but he is standing there with the same bottle of whiskey in his right hand - bottle is intact and unbroken.
In the Marshall's farm-yard: when Jake Cutter (John Wayne) introduces Paul Regret to Bessie Marshall, the sunlight on Regret's face - as well as on the horse behind him - instantly changes to overcast between shots.
During Jake's and Regret's visit to the Schofield farmstead: the angle of sunlight and lengths of shadows cast by men and horses change markedly between shots. For example, one minute Jake Cutter is talking to Major Henry in what appears to be nearly midday sunlight. The camera angle changes, and we see Jake watching Major Henry lead off the mounted team of Rangers, but now they are all casting fairly long shadows. Then the action goes back to Paul Regret, but now the sunlight indicates somewhere close to mid-day. This random shifting of sunlight and shadow lengths repeats throughout the Schofield scene.
Inside the boat, Regret is sleeping on the bed. Suddenly, in the shadow projected on the wall behind the bed, we see the door opens and the image of Cutter appears. The next shot shows Cutter himself and his shadow on the door his left hand.
Paul Regret knocks Jake Cutter unconscious with a shovel,
throws it on the ground, and escapes. When Jake wakes up later the shovel is laying the opposite way, the handle and business end have exchanged places.
When Jake and Regret arrive at the Schofield farmstead Tobe, played by Patrick Wayne,is having his left knee worked on for a previous injury. Between cuts the bandage being applied disappears then reappears.
During the last battle of the movie, Cutter is shown in a wagon shooting a lever action rifle left-handed. The film is reversed, because the loading port of the rifle is depicted as being on the left (wrong) side of the rifle.
In the final battle scene Amelung falls from the wagon with his hands tied behind his back. As he falls he places his hands in front to catch himself then quickly places them behind him again as others rush to assist him.
When Capt. Cutter handcuffs Paul Regret in his bed on the boat after as he wakes him up, one end of the handcuffs is simply resting around the narrowing end of the bedpost with nothing holding it there. To make matters worse, at the end of the scene when Capt. Cutter gives Paul Regret his suit to Don, he takes out his key to undo the cuffs as though they just can't be pulled right off the top of the bedpost.