When Dunson is standing next to his horse after Matthew Garth takes the herd from him, he clearly has a belt full of cartridges, but later on Matthew confirms to Groot that he took all of Dunson's cartridges.
(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.
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.
As the drive approaches Abilene, there is a fear that it will miss the town and eventually end up in Canada. At that time, it would have only existed as the United Provinces or today's southern Ontario and Quebec.
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 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 have been attached with dried rawhide strips.
When Tom, Matt, and Nadine are met by the two Mexicans, it is claimed that the land was granted to Diego by the King of Spain. This is not a goof. The Treaty of Guadalupe-Hidalgo that ended the Mexican-American War explicitly stated that all (former) Mexican citizen claims to land within the territory ceded to the United States would be respected.
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 his empty holster as he walks away.
In long shot as the herd is driven across the river, Walter Brennan in the wagon distant from the camera is noticeably clutching the seat with the reins passing under him to a concealed driver as the vehicle descends steeply into the water.
Tom Dunston is going to whip the sugar thief with a bull whip in his right hand, but Matt Garth shoots the sugar thief first. In the next scene, Tom has his six shooter in his right hand and not the whip.
When Dunson's wagon rolls away after leaving the wagon train, there are four cows tied to it.
Just before young Matt appears, the back of the wagon is seen with two cows, one standing and one lying down on its side. Later, when young Matt ties his cow to the wagon, there are only the two cows that the new herd started with.