Other mistake: There is a scene where a dead Indian warrior is discovered. In this scene he is covered by a large flat piece of stone. When they pull away the stone you can see his chest rising and falling as the "dead" Indian breathes. Much easier to see when viewed in HD.
Revealing mistake: In the scene right after they leave Scar's wigwam, when Ethan and Martin are speaking to each other around their camp you are able to see a small brown dot right above the sandy hills. This dot is the character Debbie's head while she is waiting for her cue before she runs in to the scene.
Continuity mistake: In the scene where John Wayne is looking down at the arrow that Look has left, his moccasin is all brown. In the next shot, his moccasin is partially opened on the side, revealing his sock.
Visible crew/equipment: The scene where Martin and Ethan watch the Calvary ride in through the river, a car or truck is seen driving up a road to the right and comes to a stop.
Continuity mistake: After the fight between Martin and Charlie just before the wedding both fighters get extremely dusty and dirty. Then Mose appears after escaping from the Indians. While the rangers and searchers gather to hear Mose, Martin and Charlie are now in clean clothes and appear to be freshly showered.
Continuity mistake: In the scene where they find the massacred cattle, Ethan tells Martin that the horses need grain and rest. Ethan and Mose ride by Martin in the next scene and Martin, whose horse is dead, tries to get a ride with Mose. Despite the fact that Mose and Ethan have horses that are galloping quickly, Martin arrives at the burning house the same time as them.
Visible crew/equipment: In the scene where Scar is sitting with another renegade in front of a teepee, you can see the overhead lighting equipment very prominently.
Continuity mistake: After repelling the Comanche charge at the river, the three searchers continue on alone, three men, three horses. Yet, after Brad makes his suicidal charge, the next morning the two remaining gents have acquired a pack horses and gear.
Factual error: The movie's beginning takes place in 1868, yet all the men pack cartridge Peacemaker Colts (not available until 1873) and model 1892 Winchesters (not available unti 1892).