Factual error: Near the end, when Cooper and Rachel are talking in the street, you can see in the background a multi-story brick and glass office building. It appears again a few shots later.
Factual error: It seems unlikely that the bullet hole in Harmonica's jacket should have burned edges - he was shot at a distance of more than ten metres. (00:36:35)
Factual error: This movie is set in the pre-Civil War Old West. At about 01:06:50, a slave, who is hoping to be taken to Mexico (where slavery is prohibited), is trying to bamboozle the lady played by Shelly Winters into believing he has vast astrological knowledge. And he points at the sky and mentions Pluto (which hadn't yet been discovered then). (01:06:50)
Factual error: When Pauline is burying her husband in the cemetery there is a shot of a mountain with vultures swarming around it and an antenna mast is visible on top of the mountain. (00:24:18)