Continuity mistake: In the scene when Billy is shooting Doc's handcuffs, Billy's revolver changes several times, shot by shot. Watch for the front sight to appear and disappear between the POV Doc shot and the medium distance shot, and for the entire finish of the gun to change at least twice.
Continuity mistake: Late in the movie, Billy is captured and jailed in an upstairs room with Bell to guard him. A knock sounds at the door and Bill raises his rifle (cocked) and points it at the door. The door is opened revealing a lady. Bell, still aiming the rifle (now suddenly uncocked), quickly lowers it.