Factual error: The action in this movie takes place in 1869, yet half of the handguns are 1873 Colt Peacemakers, and the remainder that are Army or Navy Colts are all cartridge conversions, which were not available until 1871.
Factual error: The movie takes place in 1850-51, and yet when the Rabbi is challenged to a shootout in San Francisco, the weapon he is handed is an 1858 Remington New Model Army revolver.
Factual error: As the stagecoach rolls into Prescott, Arizona, the road winds among hundreds of Saguaro cacti. Prescott is far beyond the northern limits of the Saguaro.
Factual error: Captain Stanley says of Australia "What fresh Hell is this?" The scene takes place in the 1880s, yet the quote is attributed to Dorothy Parker, who would not have been born yet.
Factual error: Ulrika is present at the communion in Karl Oskar and Kristina's home, even though she's a Baptist at this point. Not only would she not want to come, or even be welcome since she was now practicing a different form of Christianity, she would not need to come. Being married to the Baptist minister she would be attending communion on a regular basis.
Factual error: When Tuco is in the gunshop messing around with the guns, one gun has the cylinder and barrel moving back and forth by manipulating the trigger guard-lever. This is a Belgian Galand & Somerville revolver, a gun that did not exist until 1868. This film takes place during the US Civil War, which ended in 1865.
Factual error: When Chris Farley is climbing up the side of the mountain to return the bird egg, you see the bottom of his moccasins and they appear to be made of a ribbed (for traction) material. Back then, they would have been made from animal hides and the bottoms would have been smooth not ribbed.
Factual error: When the Confederates enter the plaza to be greeted by the Mexican authorities, the Stars and Bars are raised below the Mexican Flag. However, during Maximilian's reign, the Mexican Flag was completely different from the flag shown.
Factual error: The movie takes place in 1868, and every guy and his dog are wielding cartridge conversion Army, Navy, and even what appears to be Walker colts. Unfortunately, such conversions were not performed until 1869, after Smith and Wesson's rights to the Rollin White patent expired.
Factual error: The gatling gun that Josey Wales uses had the options of a 40 round magazine, or a 104 round drum magazine. Neither Josey or the Red Legs ever bothered to reload the weapon. The gun would have been empty after the 10 barrels revolved at most 10 times. Josey alone shoots for a couple of minutes straight.
Factual error: The movie's beginning takes place in 1868, yet all the men pack cartridge Peacemaker Colts (not available until 1873) and model 1892 Winchesters (not available unti 1892).
Factual error: When Gene Hackman is shot at the end he looks down and sees the hole in his shadow. However if he was shot from someone at the same level as him, the hole would be straight, parallel to the Earth's surface. So if he could he the reasonably proportioned shadow the angle of the sun wouldn't show any hole in him at all.
Suggested correction: First, it is reasonable to assume that Hackman is significantly taller than Stone, and, given that she is firing from a hip draw the angle of entry for the fatal gunshot would be on a upward trajectory. This generally upward bullet trace combined with the evening or late day sun could be reasonably expected to cast a shadow if you suspend disbelief initially to allow that this type of wound is practically plausible.
The fact that she is shorter is irrelevant given the distance between them. For the angle of the light showing through him to make sense (and that's assuming it makes sense for it to show through him at all) she would need to be kneeling directly in front of him. The fact that, as you say, suspension of belief is required demonstrates the original mistake is valid.
Suggested correction: He doesn't see the hole in his shadow. We see him look downward slightly, then we see the shadow slightly from the side, not the view he would see. We then see him looking down at himself. It's easy to assume he is looking at his shadow, but we don't see him doing that. He could simply be staring into space, with the shadow presented for dramatic effect for the viewer.
Factual error: When Jimmy Stewart is almost down to the runway during the bad weather, the aircraft exterior shots show the type of cloud tops found only at high altitude, not low to the ground.
Factual error: John Blair rides for the pony express, which operated until October, 1861. However, all of the handguns in the movie are cartridge Colts, which did not come into existence until 1871.
Factual error: The movie "Santa Fe Trail" (1940) starring Errol Flynn, Ronald Reagan and Raymond Massey, is the story of John Brown's raid on Harper's Ferry in 1859. The president is shown to be Abraham Lincoln, who wasn't inaugurated until February of 1861.
Factual error: While the film is supposedly set in the late 19th Century, the locomotive is oil-fired. The tender carries neither wood nor coal. There is no way the Mexican National Railroad would possess oil-burning locomotives in this time period.
Factual error: Campbell is armed with what is described by Wayne as a "buffalo gun", a large bore rifle known for its power and substantial recoil. Campbell shoots a wild turkey with the gun and causes the meat to be severely damaged. Wayne comments, "Too much gun" emphasizing the power of the gun. However, when Campbell shoots the turkey with this "buffalo gun" there is no visible recoil.
Factual error: The opening scene shows Spencer Tracy on a crack streamliner in the desert east of Los Angeles, featuring red and orange coaches. Such equipment was used by the railroad on only two trains, neither of which crossed the desert east of Los Angeles, and both of which had matching red and orange locomotives. In the film the train is pulled by a dark gray locomotive normally used only to haul freight, and in a close-up shot the number in the locomotive is preceded by an X, which in the language of railroading refers to an extra train.
Factual error: Two grave markers at the end, approximately one hour and 27 minutes into the film, have the date of November 7, 1889, on them. The script made it clear that the climactic gunfight between Glenn Ford and Broderick Crawford came on a Sunday. Ford was called out of church services, for example. According to The World Almanac and Book of Facts, November 7, 1889, was a Thursday, not a Sunday.
Factual error: In the beginning during the opening credits, Bud is using a map to find his Uncle Bob's house in a new housing addition. A new home in a new addition (where they are still building), would not be on a paper map yet.