In the opening scene, Cable is pointing a Henry rifle, which has a thick barrel and small bore, at Bowen and Taggart. Bowen then wrestles the rifle away from Cable, and as Bowen points the rifle at Cable, it is now a thinner barrel and larger bore Winchester.
At the end, as Joshua begins riding away on his motorcycle, there are two horses in the corral. As Joshua rides around the other side of the corral, the horses are gone.
When Cable leaves the land claim office, he walks in front of a black horse facing right. In the next shot, the black horse is facing left.
In the opening scene, Hogue confronts a Gila monster. In the first shot, the lizard is on a large rock with its front left leg and claws slipping over the side of the rock. After cutting to a shot of Hogue, the view returns to the Gila monster, which is now standing in the middle of the same rock with no time or ability to have moved.
When Hildy kicks through her door, the hole is a different shape on the inside (jagged border) compared to the outside (smooth border).
Hildy is giving Hogue a bath, and he gets out. As Hogue is wrapping the towel around him, his underwear can be seen on both sides of the towel.
Before Hildy kicks a hole in the door, the the shape of the weakened section can be made out from the outside.
The flag given to Cable has 50 stars although the US flag in 1908 only had 46 stars.
When the Rev. Sloan is comforting Claudia and unbuttons her blouse, it's obvious that her skirt has a zipper. The movie takes place in 1908, but the modern zipper for clothing wasn't developed until 1913 and patented in 1917.
When Hogue pours the pitcher of water on Quittner's boot (at about the :30 min point) and slams the pitcher down, the sound reveals it is a plastic pitcher, not glass, decades before it would have been commercially available.
When Bowen is in the sand pit, his hat is shot off by Cable Hogue. However, the gunshot sound comes significantly after the hat flies off.