When they are driving the cattle and go to the aid of the wagon train, a communications mast can be seen on the hill in the background of one shot of the five horsemen. The next shot is a closer view of one rider and the mast can be seen even more clearly.
(at around 40 mins) In the poker game scene, a man is playing the guitar and singing. The guitar he is playing is a carved-top, F-hole guitar. This type of instrument would not be introduced until about 1900, decades after the movie is supposed to take place.
During the cattle stampede, Dunson, Matt and the other cowboys saddle up and try to turn the herd. Process shots of each cowboys are inserted in the scene. Every cowboy is riding the same dummy horse and saddle with a very large Mexican saddlehorn.
When Dunson is standing next to his horse after Mathew Garth takes the herd from him, he clearly has a belt full of cartridges, but later on Mathew confirms to Groot that he took all of Dunson's cartridges.
At the beginning of the film, Dunston's wagon is shown going to the left of the wagon train, which is correct for a train going to the west. Later, when they stop at the Red River, Dunston looks back and sees the fire from the wagon train. He looks back and left of where they are. If they were moving south, the train should have been back and right of Dunston's wagon.
When the Mexicans ride up and Dunson asks them the name of the river, they reply without hesitation, "Rio Grande." The river has always been called "Rio Bravo" in Mexico, which is what they would have answered.
In the fight scene at the end the cross bar on the hitch rail is steel pipe which was not found in the 1860's. The rail would have been made of local wood and would normally been attached with dried rawhide strips.
Matthew Garth seems to beat Thomas Dunson to the draw in a mock contest orchestrated by Dunston and Groot Nadine. However the gun is already in Garth's right hand in order to fool the audience, as evidenced by the his empty holster as he walks away.