Continuity mistake: When the church bell is rehung and struck, it goes from shoulder height to well above head height, then back to shoulder height in subsequent shots. (01:28:15)
Factual error: As the 7 are coming into the camp, it shows an American Flag flying all 50 stars. In 1879 there were only 38 stars on the flag.
Suggested correction: It's a 37-star flag, the top and bottom row have 8 stars and the middle 3 rows have 7 stars. While Colorado was admitted into the Union in 1877 as the 38th state, it's not unreasonable to think they just didn't get an updated flag. However, it's also possible whoever was in charge of getting/making the flag got it mixed up since the 38-star flag has 7 stars on the top and bottom row and 8 stars in the middle 3 rows.
Suggested correction: Recheck the flag stars. They appear to aligned and not offset, as in the modern flag.
I saw this also and think it might have been a 48 star flag.
Continuity mistake: The number of men killed in the initial gun fight is more than the 22 men Bogue says worked for him.
Continuity mistake: Chisholm says Powder Dan kills the wife of the tenant farmer. Then, after he kills Powder Dan, he tells the town sheriff to send half the reward to the tenant farmer's widow, who he previously said Powder Dan took advantage of and killed.
Continuity mistake: Towards the end of the movie when Bogue's men are first charging into Rose Creek, we see white tents camped on the outskirts of town in the field. The next wide shot from a distance there are no white tents, then the next shot there are. In the final scenes of the movie as the camera pans over the town and into the field, there are no white tents visible.
Revealing mistake: In the Gatling Gun scene when the camera cuts to a short couple frame shot showing a closeup on the .45-70 Government ammunition casings on the ground that are falling from the gun, the casings are star crimped. Star crimped casings indicate blank ammunition. Standard ammunition isn't possible with star crimping. The size and casing shape indicates .45-70 Gov which was popular in these guns at this time. (01:46:40)
Factual error: In one of the opening scenes of the riders heading toward the town you can see contrails of aircraft on the lower left horizon.