Other mistake: Bandoleer rounds don't fit any weapon in the show. When Pierce tries the Gatling gun, there is no magazine in it.
Plot hole: Towards the end of the film when Newman and the remaining stagecoach people are in the cabin at the top of the hill, Richard Boone's character sends one of his two henchman (the Anglo, not the Mexican) around to the back of the cabin to apparently surprise them from behind. Then Newman's character walks down the hill and Boone and the Mexican henchman are all killed in a shootout. Everyone comes out of the cabin, comes down the hill and the credits roll. I guess that now, almost 40 years later, that poor Anglo-henchman is still behind the cabin, waiting for them to come out.
Factual error: Upon completing the task of lowering every wagon, horse, mule, ox and human down to the canyon floor, the settlers proceed to build rafts for their wagons..however, their scout would have informed them that they would be going overland to intercept the Columbia River to the northwest. The Snake River, with class IV rapids, (even now tamed with dams) was no place for the uninitiated to go rafting.
Other mistake: The villain's plan is to impersonate along with his large crew a platoon of the Mexican army. He already captured the soldiers and has them incapacitated, all he needs to do is take their uniforms. Instead of doing so, though, he mows down the whole lot with a heavy machinegun. When it's time to put on the show, everyone wears nice and tidy white uniforms.