The color of Molly's horse changes.
Joel McCrea is wearing a jacket in the bar when he learns that Brian Dunlevy wants to see him. He wasn't wearing a jacket when he left his fiancée in the hotel lobby.
Before the shootout at the end, both men see that the sun is about to set. And yet the shootout occurs in full sunlight; and when the newlyweds ride off at the end, the sun still hasn't quite set: they are riding off into the sunset.
Upon returning from the "wild" ride on the unbroken horse, the Virginian turns the lathered horse loose in the corral with the saddle still on. No experienced horseman would do that, because one of the first thing the horse would do is roll on the ground, thereby ruining the saddle. What should happen is the rider would unsaddle it, lead it around the corral until it cooled down, brush it and give it water; but not too much, because it could cause the horse to become sick.
Ranch work is a hot, sweaty business, entailing many long hours in the saddle and afoot. But, in this movie, almost every man, whether working or walking about town, is wearing what looks like a new hat and clothes, neat and clean, with no sweat rings or stains.
When Molly first arrives at her cabin, she hears an animal howling. Mr Taylor says it is a coyote. But what we hear is actually the howl of a wolf. A coyote's cry is a barking, whining sound.
The Virginian tells the school teacher that he will "learn" her how to ride western. She explains that he should say "teach" because it is a transitive verb while "learn" is intransitive. This is not correct: learn does get used as a transitive verb (e.g., "I learned what you taught me").