Factual error: In the scene where Lee Marvin states that Burt Lancaster loves explosions, Burt Lancaster makes a remark about the big bang theory. He says: the whole universe started with a big explosion. The film is set in 1917 which is 10 years before Georges Lemaître launched the big bang theory.
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.