The Winchester model 1866 was nicknamed the "Yellow Boy" due to the brass receiver. The Winchesters used in the film were actually from 1873. Since 3 years after the war was only 1868, Ethan and the others couldn't have had those model 1873 Winchesters.
At the Comanche camp where Martin buys a "blanket" the female extras are wearing clothing, jewelry and hair styles typical of 20th Century Navajos (not surprising, as Monument Valley lies within the Navajo Nation.)
The movie begins in 1868; however, all of the guns used are
mid 1870s vintage. The pistols used are Colt 1873 Peacemakers and the rifles are Winchester Model 1892. Although both Spencer and Volcanic Repeating Arms both produced cartridge firing repeating rifles, it is more likely that most of the men, not being professional gun hands, would have carried Civil War surplus rifles (muzzleloaders) like Mose.
Laurie is seen wearing what appear to be riveted blue jeans in Texas around the year 1869 or 1870. Denim pants reinforced with rivets were not patented and mass produced until 1873 (by Levi Strauss in San Francisco), and prior to that were likely unknown outside of Reno, Nevada, where they had been invented by a local tailor.
Although the movie is set between 1868 and 1873, several characters are seen wearing Levi's jeans (complete with stitching on the back pocket and signature red Levi's tag). Levi's didn't patent a blue jean until 1873, and the name "Levi's" wasn't patented until 1928.
During the shootout with the Indians at the river, Rev. Capt. Clayton's gun is emptied and Ethan throws him a loaded gun; Clayton throws his hat and hits Ethan. Ethan is moving his lips and saying something but, there is no sound.
When the Comanches ride down on Ethan and Marty from the sand dune, one Comanche can be seen and heard firing his rifle. However, he fires his rifle a second time but no gunshot sound effect can be heard.
When Rev. Capt. Clayton is swearing in Martin and Aaron as Deputy Texas Rangers he is continually interrupted. One interruption occurs immediately after he says, "...and you will faithfully discharge..." After the interruption he asks young Debbie where he was, and she replies, "You will faithfully fulfill." When he restarts the oath, Clayton again says, "You will faithfully discharge..."
When the Indians charge across the river toward the reverend's posse, the river changes both direction and color throughout the scene. In some shots, the river is a muddy red, while in others it is clearer and blue. In the shots of Ethan firing his rifle, the river moves from his left to his right. But in the shots of the Indians getting shot and falling from their horses, the river moves from Ethan's (and the viewer's) right to his left. Then, as the Indians retreat, the river switches back to moving left to right.
During the battle with the Comanche crossing the river, Mose and Martin seem enveloped in dusk while firing from behind a log, whereas their companions, seen in other shots, are viewed in bright sunlight.
When Martin is taking a bath, Laurie brings in 2 buckets of water and leaves them by the door. Then when the camera cuts to a close up of Martin in the tub, the buckets are next to the tub. When the camera pans back to a full shot, the buckets are by the door again.
The horses Ethan and Marty are riding are not in sequence. When Marty rides his horse to death and is seen carrying his saddle, he was riding his buckskin (which is the horse that supposedly died). When he rides out with Ethan to look for the two girls, he is again on the same buckskin horse. Also, when Marty leaves and goes after Ethan, Laurie gives him her blaze-faced sorrel 'Sweet Face'. When Ethan and Marty are riding together after Marty buys 'Look', they are both riding dark faced dark horses and not leading any other horses. Later on, Marty is once again on 'Sweet Face'.
When Clayton's posse is crossing the river, you can clearly see the Indian war party following behind on the river bank. When the posse gets to the other side, the war party is not visible on the opposite bank or in the river.
When Clayton's posse is about half way across the river, the scene cuts to a close up of five Indians coming over a dirt bank. Seconds later, a puff of smoke is seen coming from the bottom of the screen when all five Indians fall off their horses and into the water (the ropes used to trip up the horses are visible in the muddied water). The posse is already across the river and too far away to make such a shot; one shot cannot bring down five Indians.
As Ethan and Marty approach the Jorgenson ranch: when Mrs. Jorgenson is standing outside the front door, the position of the sun changes from shot to shot, in the first her back is in direct sunlight with her shadow in front of her, in the next shot it is now the front of her body that is in sunlight, now her shadow is behind her. In the next shot the position of the sun has reverted back to the position it was in the first.
When the fight between Marty and Charlie is broken up by the wedding guest, both men are completely covered in white dust. When they cut to a closer shot of the two men, they are much cleaner with only a small amount of brown dust on them.
At the Jorgenson ranch when Laurie is cooking breakfast, the bottoms of the legs of the jeans she is wearing are turned up. In the next scene where she hands her horse over to Marty, the bottoms are no longer turned up.
When Ethan and Martin are left by Look, the day dawns bright and clear and they are lightly dressed (with Martin with his shirt open), but in the "letter" Martin sent describing that day, as read by his girlfriend in a quick cut, Martin says it "snowed that day," and the next quick cut shows them riding through a snow covered landscape, which can only mean that a warm, sunny day they woke up to, to find Luke gone, very suddenly turned into winter.
At the beginning, when Ethan takes his brother's place in the Ranger posse, he walks out wearing his chaps. When he gets on his horse and rides out, his chaps are laying across the front of his saddle.
When the Comanche attack is imminent on the Edwards homestead the window's wooden reinforcements facing out from the front of the house go from being closed/boarded up when viewed from outside to open when the character Aaron Edwards comes back into the house. He then closes them again.
Just after the shooting at the river Ethan is sitting and putting new bullets in his rifle while talking to Clayton, then he walks up to the two by the horses behind with his rifle in his left hand. Just before the cut to a closeup on the three, he shifts the rifle to his right hand. But after the cut to the closeup he is still holding his rifle in his left hand.
When the Reverend breaks up the fight between Martin and Charlie, a group of men are seen behind the Reverend with Martin and Charlie in front of him however, when the fight resumes, Seth, the violinist, has appeared behind Charlie.
When Ethan, Marty, and Mose meet Emilio Gabriel Fernandez y Figueroa in the bar, the bartender has put out an inferior bottle of tequila. Emilio asks (in Spanish) for a better one but the bartender only wipes off the same bottle and places it back on the bar. In the next shot, a different bottle - with supposedly a better quality tequila - is shown.
When Ethan walks up to the stone "arrow" marker left by Luke when she runs away, his moccasins have high tops above the ankle. Then, when you see him kneeling at the arrow, his moccasins are rolled down, revealing modern sweat socks.
When the men are going after the cattle thieves, Ethan puts on his gun belt and buckles it. When the camera cuts to a medium shot, he is putting the tongue of the belt, through the buckle after he's already completed buckling it in the previous shot.
After Ethan stokes the campfire and Martin turns in for the
night, the camera tilts up to Futterman on a rock. The angle is so high that the top of the studio backdrop and a studio light are visible when the film is projected or seen in 1:1.33 aspect ratio.
In the cabin where the cavalry is holding the deranged woman
and the two teenage girls, a filming light is visible in the ceiling after the deranged woman screams and grabs the doll. The light is partially hidden behind a horizontal stovepipe and the glare is clearly visible for most of the scene.
In the long tracking shot of the calvary riding through the Comanche village near the end of the film, dust kicked up by the dolly riding on its track is visible at the bottom right corner of the screen.
The Native Americans were supposed to be Comanche, but virtually everything about them was Navajo or generic Hollywood "Indian"--nothing distinctly Comanche. They speak Navajo throughout the film (a common word is "yatahey" i.e. "hello").
At the beginning of the film, when the Rangers discover the prize bull and decide that it is a "murder raid", Martin rides off in the same direction as those going to the Jorgensen ranch (west) instead of heading south, towards his family's place.
Ethan's medal, which he gives to Debbie, is a prop combining features of French and Mexican medals. The white and blue Maltese cross is similar to the white and red cross on the Mexican Order of Guadalupe and the red and green ribbon appears on the French Order of Merite Agricole.
As Ethan and the Rev stand together shooting across the river, Ward Bond, the actor playing the Rev fires his pistol prematurely, before it is lowered. Etan and the Rev seemingly fire simultaneously, but that is when the Rev fires high, and continues to lower his gun into an aiming position. But then the camera cuts to a closeup of Ethan, aiming his Winchester. It is evident the actor is just pulling the trigger with nothing to aim at.
Ethan and Mose are on horseback and pass Martin who is on foot during the return to Aaron's ranch. Martin arrives at the burning ranch only seconds behind Ethan and Mose, but should have been hours behind.
When Laurie receives the letter from Marty she throws it into the open fire in disgust. It's obviously supposed to catch fire but it doesn't. Her father, Lars, quickly kneels down and can be seen setting light to the paper before dropping it on the hearth and stamping on it to put the flames out.
The outdoor scenes set at night (those not shot in a studio) are clearly filmed during daytime. Highlights and hard shadows are an easy giveaway. By underexposing the film it is then made to look dark.
The goof items below may give away important plot points.
When Brad returns to Ethan and Marty after scouting the Comanche camp, he tells them that they are camped a half mile away. After learning of Lucy's fate, he mounts his horse and rides off towards the camp. About five to seven seconds after his departure, we hear the sounds of gunfire. For a good saddle horse, carrying an average sized man and saddle with gear, to run a half mile at full speed would require roughly 50 to 60 seconds lead time. Brad could not have come remotely close to the Comanches in the time between his leaving and the sounds of gunfire.
When Ethan and Marty finally find Debbie she has grown from a child (Lana Wood) into a young woman (Natalie Wood), yet the only sign of aging amongst the other members of the cast is a simple streak of gray in Ethan's hair in the wedding scene.
When Lars Jorgensen is trying to stop Ethan and Martin from entering his house just before Laurie's wedding, their shadows can clearly be seen on the painted backdrop behind. A few minutes later, when Martin and Charlie go outside to fight, their shadows are also briefly seen on the backdrop.