Revealing mistake: When Tuco first enters Sad Hill Cemetery, you can see the dual tire track ruts left by the dump truck that brought in the sand to make the grave mounds. (02:36:35)
Factual error: The date on Arch Stanton's grave is "3 February 1862," which is how dates are written in most countries. However, in the US, it would be written "February 3, 1862." (02:39:30)
Other mistake: In the very last scene of the movie, and just before Blondie shoots the rope that is holding Tuco on the grave's cross. You can see a car moving in the background, screen right of Tuco's head. (02:55:29)
Suggested correction: I've just watched this scene; there's no car visible. There's a bit where Blondie prepares his gun prior to aiming, then the camera switches back to Tuco. To the right of Tuco, in the background, a blurry something can be seen for maybe a second moving right to left. The object is far too blurry to identify. There's no reason to believe it's a car. It makes more sense that it's a horse-drawn wagon.
Other mistake: When Blondie and Tuco are talking about telling each other the location of the money, a car drives by in the background. In the shot where both Blondie and Tuco are visible, watch the top right in the trees in between the pillars supporting the bridge.
Other mistake: Director Sergio Leone uses one of his trademark filming techniques throughout this film, where anything outside the camera frame doesn't exist. Several times the characters react to situations and other characters when they come into frame that they logically would've seen much sooner. For example, at one point Blondie and Tuco are walking and stumble across an army camp. Even though in reality they would have been walking toward it for a long time and would have been aware of it, they react with surprise the moment it appears on screen.
Factual error: The guns Tuco and Blondie use appear to be 1851 Colt Navy because they have octagonal barrels and the loading ram below the barrel. 2 problems are that 1) the '51 was only made in .36 calibre. When shown from the business end the muzzles appear to be much larger. Colt '61 / '60 came in .44 caliber, but they had round barrels. 2) More importantly they are using brass cartridges to load. Conversions for those pistols to load cartridges did not exist till after the war and these were cap 'n' ball only then.
Factual error: Regarding the train scenes when Tuco is taken then escapes from Mario Brega, there were no trains / railways in New Mexico in any of the years of the US Civil War, let alone 1862 when Selby's short-lived and daft campaign was limited to. I believe the RRs were starting to move into Texas then, but not NM until the 1870s. Also, the whole geography of the film is wrong if the gold was intercepted but 'escaped' with the three people in the wagon near Glorietta Pass in early 1862. The sequence of events means all involved had to go up and down the Rio Bravo constantly - eg. the 'jornado del muerto' is hundreds of miles from Glorietta. This is too complicated to summarise here, but check the actual battles and timelines.
Factual error: The movie depicts General Sibley's New Mexico campaign. This occurred in the first half of 1862, with the last battle occurring on June 1, prior to a Confederate retreat to Texas. General Sibley is seen, and he lost his battlefield command altogether in 1863. However, Angel Eyes makes reference to Andersonville prison, which did not open until 1864. The question is also begged as to how Arch Stanton, who died in February of 1862, had completely rotted away by May in dry soil.
