Revealing mistake: When Howard and Curtin chase Dobbs, they pass a dead donkey which is obviously breathing, and the rope used to tie its legs together is visible too.
Revealing mistake: Before the first encounter with the Apache, O'Rourke Jr. is looking out for Indians hiding behind a trunk. When he is being shot at and two bullets hit the trunk you see tiny volcanos erupt which obviously have been filled with explosives. (00:57:00)
Revealing mistake: When Potter starts shaving at the morning of the Indian attack on the wagon train, his mirror falls off the post, and as he bends down to retrieve it, an arrow embeds itself right above his head. If you look closely in that scene, you can see a length of targeting wire used to guide the arrow sticking to the post above Potter's head. (00:37:10)