Continuity mistake: After the Baxter/Rojo dinner, the background changes from nightime to daytime at the cemetery, then back to nightime again when Clint kidnaps Mirasol.
Factual error: When the "U.S." soldiers open fire on the Mexican cavalry, they employ a machine gun. The only two rapid-firing weapons in that time period were the Gatling gun, which used a magazine or drum of rounds and was fired by turning a crank, or the "Mitrailleuse" volley gun, which required at least two men to operate it, one to load the ammunition blocks, and the other to turn, once again, a crank to fire the weapon. The gun pictured has a single operator and neither a crank, nor any visible cartridge feed.
Joe: When a man with .45 meets a man with a rifle, you said, the man with a pistol's a dead man. Let's see if that's true. Go ahead, load up and shoot.