When Tony and Pat escape from the Sheriff's Office during the stampede the coat rack is standing upright. Yet when Doug returns later the coat rack is leaning up against a wall.
While fighting in the cabin, Doug gets flipped over the sink and the pump comes off without any resistance. In reality it would be connected to a metal pipe and shouldn't come off that easy.
Billy the Kid is portrayed as left-handed, which is a common misconception. William Bonney was ambidextrous, but primarily right-handed. The only known photograph of him gives the incorrect impression that he was left-handed because his revolver is positioned on the left side, but the image was later found to have been flipped.
During the stampede through town, there's one scene where 3 rows of barbed wire can be seen keeping them in place.
When Tony and Pat arrive at the ranch and are dismounting from their horses, Tony says "I don't know." but his lips don't move.
When the deputy is trying to calm the mob outside the Sheriff's office, the sign on the window says "Pat Garret, Sheriff". The correct spelling was Garrett.