Continuity mistake: There is no way the cow (a Hereford I think) could have given birth to that calf (a Jersey or something like that). [The mom is a mixed-breed BEEF cow, the calf a purebred Jersey DAIRY calf. An amazing feat of animal husbandry.]
Continuity mistake: In the barn dance scene Gideon is doing flips on the sawhorse. His shirt becomes untucked and then retucked between flips.
Factual error: When Bob Ford opens the box under the bed, on top is an issue of "The Jesse James Stories" weekly, which was published from 1901-1903 - fully twenty years after the events of this film. (00:36:10)
Revealing mistake: In the scene out West with the buffalo, notice the car driving in the distant background.
Suggested correction: What is the time code? In the scene with the buffalo stampede there are no cars anywhere.
Continuity mistake: When Jack Twist is chopping wood, right before Joe Aguirre visits him on his horse, he sets up a piece of wood to chop. Jack talks to Joe and when the camera pans out to show him ready to chop again, the piece of wood is gone.
Visible crew/equipment: When the young lady is trying to drag her dead husband away to bury him, as she takes a hold of the blanket you can see the covered wagon in the background and someone's legs behind the wheel begin to move to the right. She was suppose to be alone except for her dead husband. (00:34:00)
Factual error: When the riders are watching the wagon train, in the background you can see a communication tower with conical speakers.
Revealing mistake: As Jango fires the machine gun, the ammunition chain hanging from it is loaded on both sides, and it doesn't move. Also the belt has different sizes of cartridges in it. (00:45:30)
Character mistake: When Vallian prepares to leave the homestead, McKaskell asks him how his chest wound was. Vallian was shot low on the right side, just above the gun-belt, and McKaskell, who removed the bullet, was quite aware of the location.
Factual error: The cavalry sing Low Bridge (or Fifteen Years On The Erie Canal) as they trek across Texas in 1879. This was written by Thomas S. Allen in 1905 during the time the Erie canal was switched from mule power to engine power. (00:49:00)
Continuity mistake: At the start of the train chase Tonto blows the whistle as he passes the bad guy who says "shoot him you idiot". The soldier opens fire with the machine gun and Tonto is suddenly back in front and passing him again. (02:03:35)
Other mistake: Bandoleer rounds don't fit any weapon in the show. When Pierce tries the Gatling gun, there is no magazine in it.
Factual error: Outside the Territory Hotel, during the storm, and once earlier in the movie, the taxi cabs are shown with the modern "flip over" vacancy/hired signs that were not used then.
Continuity mistake: Near the end after the mob goes over the cliff and Jim is chasing the horses through a snow patch, another rider on a light colored horse can be seen on the right. (01:36:00)
Continuity mistake: In the first shot of the dead man hanging form the tree, his head appears to be about 12" from the tree limb. After Mattie climbs up to cut him down, his head is several feet from the tree limb.
Continuity mistake: When the gang of bad guys ride to the ranch there are nine of them. When they leave, and throughout the rest of the movie there are only seven, despite only one being shot off his horse at the ranch. (00:15:15)
Factual error: The character Red Harvest is a Comanche. He is sporting a Mohawk haircut. This hair style was not in use by the Comanche tribe, but used by the Iroquois and Mohawks.
Suggested correction: The Mohawk hairstyle was used amongst many different plains tribes as well as eastern tribes.
Other mistake: There's a horse that has apparently ridden up a cliff, and an electrical burning brand that can be changed onto different flame settings.
Continuity mistake: At the beginning, in the scene where Rayne finds two bodies near a small house, she takes out one of her blades. Firstly, she grabs it in a wrong way (the tonfa-like blades should be held under the arm), but when the camera angle changes she holds it properly.
Continuity mistake: A couple of times in the movie, the camera pans to John Wayne, and his hair is parted on the wrong side. (Reverse editing flaw).