Other mistake: At the beginning of the film, you see Colonel Mortimer traveling on a train while reading a bible. He has a Indian squaw and a gentleman wearing glasses and a derby sitting across from him. A conductor comes by to pick up their tickets. Colonel Mortimer asks the conductor, "is it far to Tucamcari?" The conductor replies, "we pass there in three to four minutes." The gentleman with glasses starts to tell the Colonel that he is on the wrong train. As he tells him this, look out the train window and see a car driving off into the distance. (00:04:30)
Other mistake: When Jimmy Stewart talks to George Kennedy about the taking of his son as prisoner. Right after they look at the map. Look over the shoulder of Kennedy in next few frames. You can see a white vehicle move across the horizon under the mountains. Looks like a white van or truck. With a HD TV, you can even see the dust trail as the vehicle moves left across screen.
Other mistake: John Elder whacks Curley (George Kennedy) across the face very hard with an axe handle as Curley was trying to drown the undertaker. Despite this, Curley never shows any mark at all in the subsequent scenes.