Other mistake: Bandoleer rounds don't fit any weapon in the show. When Pierce tries the Gatling gun, there is no magazine in it.
Continuity mistake: While every other actor in this movie wears the same costume day and night, day after day, John Wayne's costume changes regularly, even between shots. For all daytime scenes, he wears an orange long-sleeved shirt with a contrasting blue neck-kerchief; in some night-time scenes, he wears a blue long-sleeved shirt with a contrasting yellow neck-kerchief; and sometimes his daytime neck-kerchief changes from blue to yellow in a matter of seconds, between shots.
Visible crew/equipment: After explosion and the flour wagon runs driverless spilling the flour barrels you can see a man through an opening under the seat driving.
Factual error: The highly unstable explosive nitroglycerin plays a crucial role in the film, as it is painstakingly stolen and then used to obliterate a wooden bridge that separates the War Wagon from its armed escort. However, at the end of the film, a single bottle of this incredibly volatile stuff somehow survives to be stolen by an Indian brave who haphazardly snatches it (thinking it is a bottle of whiskey) and scampers away with it. Any one of the Indian's actions, from roughly snatching it to leaping off the wagon with it and impacting the ground, should have detonated the nitroglycerin. The leader of the Indian war party even chugs a mouthful of the Nitro and violently spits it out, the action of which should have exploded his head. But the nitroglycerin doesn't explode until the Indian leader finally throws the bottle to the ground.