Goofs

The snake that crawled out of the bag after Two Cards' death looked like a garter snake instead of a rattlesnake.

When Vittorio is crossing the river, two claps of thunder can be heard, but the bright sunlight indicates that there are no clouds in the sky. EDIT: During Summer Monsoon in the Desert Southwest, it is common to hear thunder from storms that are miles away while the sky is clear and sunny at your location.

In the first scene, some of the slain Indians show signs of movement (breathing, etc.).

When Billy attempts suicide with his pistol, he pulls back the cock and closes the frizzen. He then puts the pistol to his head and is only stopped when the Apache warrior snatches the pistol out of his hand. BUT, he never primed the pistol by putting powder in the flash pan. This means that had he pulled the trigger, the pistol would have only sparked. With nothing in the pan, the sparks would have had nothing to ignite, and nothing would have flashed. If nothing flashed, then there would have been no flash to travel through the touch hole and ignite the main charge (and send the ball down the barrel and into his head). Someone of this era would have know better, and would have primed his pistol if he intended to use it (whether for self-defense or self-destruction).

When Benji has a change of heart about leaving his brother behind, the men return to Billy. Benji finds Billy's pistol, picks it up and exclaims that it is still primed. BUT he never pushes the frizzen forward to see if it is primed (if there is powder in the pan). He would only have seen the frizzen was back and covering the pan. Without pushing the frizzen forward to expose the powder, he could not have known if it was or was not primed.

With the final fight between Pitcalin and the Apache warrior, the woman primes the rifle without pulling back the cock. The cock in the forward position would have impeded the priming of the rifle as the cock would been blocked the flash pan. To prime the rifle, the cock is pulled back first. Then the pan is primed. Finally, the frizzen is pulled back to cover the pan, thereby readying the rifle for firing.