Character mistake: The bartender in El Dorado was John Mitchum - Robert Mitchum's real life brother. "Elmer" was the character name of the bartender, as mentioned twice in the first saloon shootout mid-way through the movie and in the movie credits under "Elmer, Jason's Bartender." Robert Mitchum mistakenly calls to the bartender, "freeze it right there, Johnny" (inadvertently using his brother's real name, "John, " as opposed to his character's name, "Elmer") in the final saloon shootout scene. (01:20:59 - 02:00:22)
Character mistake: At the confederate prison camp after they are captured a soldier comes up to the sergeant (Van Cleef) and informs him that the "Captain" wants to see him right away. In the next shot with Van Cleef where he is talking to the commanding officer you will see that he is not a captain but a major. (01:35:00 - 01:35:29)