Factual error: Though extremely modest on today's standards, the dress worn by Clara to the hoedown shows far too much cleavage for the time. No schoolteacher would ever wear a dress like that in the 1880s.
Plot hole: There is a barely credible explanation for the fact that a guest cannot be injured or killed by being shot in Westworld, but what about the vicious fistfight we see in the bar? People are injured or killed in bar brawls all the time, and this one was incredibly violent. How do they prevent guests from being injured or killed by the cutting and stabbing weapons we see in Medieval and Roman World? Guests are supposed to fight each other, not just robots - they cannot be 'programmed' to lose! Delos is going be sued into bankruptcy within a week of the first guest arriving. Quite apart from the legal position, think about the bad publicity! Who is going to pay the huge fees demanded by the parks owners when the media is constantly reporting on the guests who wound up dead or with life changing injuries?
Other mistake: During the final shootout in the saloon, young Joey yells, "Shane, look out!" Alan Ladd whirls around and his gun goes off. But the gun isn't pointed anywhere near the bad guy who is standing on the second floor balcony. Shane more than likely shot the furnace that was off to the right. Yet, the bad guy still manages to do a face plant on the barroom floor.
Other mistake: The film is about a gang of Confederate soldiers after the Civil War, so a black man in the gang would seem a bit out of place.
Character mistake: On the day Rebecca arrives home, Junior Douglas says that he was "Valedictorian, Class of '95." Earlier, Drago talked of almost killing Running Buffalo "during that trouble back in the '40's," which would have been about 50 years earlier. That would mean Drago and McLintock were fighting Indians when they were babies, or possibly before they were born!
Factual error: In the scene after the Mandingo fight in the Cleopatra club, when Leonardo DiCaprio gives his slave a beer, it is served in a flip top bottle. The first flip-top closure was created by Charles de Quillfeldt in the United States, who filed for a patent on 30 November 1874, receiving patent number 158406.
Visible crew/equipment: At the beginning of the film when being rescued from the quicksand you can see someone's legs and hand behind the horse, holding it. (00:10:00 - 00:11:00)
Continuity mistake: When Curly Bill is shooting up the town while drunk, he fires about 10 more rounds than his six shooter can hold without reloading (so too many shots even if he had a second gun), and still has a round remaining when he shoots Marshal Fred White. In the 1880 court hearing held after Marshal White's death, Curly Bill still had 5 rounds remaining in his pistol.
Factual error: After the birth of the baby, James Stewart and "Doc" Witherspoon are discussing the course of the war. "Doc" relates that he has always lived in Virginia and that his son died at Gettysburg at Little Round Top. No Virginia regiments fought at Little Round Top - only Alabama, Texas, Georgia, and Arkansas regiments.
Factual error: After the first encounter with the Apaches, the covered wagon is headed to the Fort at full speed. The camera pans to the Apaches on the ridge and you can see a truck driving at the bottom right corner. (00:32:00)
Continuity mistake: Indians are on a hilltop and a jet vapour trail can be seen in the sky.
Factual error: The evil man request 100,000 pieces of gold for ransom. Each gold piece looks to be about 10 ounces, and 100,000 of those would make the entire thing weigh about 31 tons, but 2 guys are carrying it around in a little chest throughout the movie.
Other mistake: At the beginning when the lawman is cutting down Eastwood from tree in the background you can see a white vehicle driving along a road in the background.