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: In one of the final scenes where Tom Selleck has a gunfight with Alan Rickman, the time of day during the few minutes (in movie time) the scene takes place varies from noon (full sunlight) to dusk. Just prior to the gunfight, when Selleck is getting ready to draw on Rickman and his two henchmen, there are almost no shadows and there is bright sunlight indicating it is in the middle of the day. Just after the gunfight there are now long shadows on the characters and moments later the camera pans across the hills and the sun is setting.
Continuity mistake: When Kevin Costner gets hit on the head by the top frame of the door, he is knocked out unconscious. Yet, when he gets back up after regaining consciousness, the blood from is head had been running down his nose. Don't you think that as he was lying down in a horizontal position, the blood would run down over the eyes towards the ears?