Senora Devereaux's dress changes colour in different shots as she stands at the gate talking to Matt Devereaux.
Twice in the film, Arizona is called a "state". The film is set in the 1880s. Arizona became the 48th state in 1912. In the 1880s, Arizona was still a territory.
At the beginning of the film after Joe is released from prison, his brother Ben tries to bribe him to go to Oregon, which he says is "a brand new territory". Oregon was granted Statehood in 1859, over 50 years before Arizona became a State.
When Ben pulls the "dead" Matt from his horse, Matt bends his right knee and swings his right foot to clear the cantle.
A lance that easy to break wouldn't be much of a weapon or hunting-tool.
When Barbara visits Joe in prison, he angrily leaves her, and when he grabs the iron bar cell door, it visibly shakes and wobbles.
When Matt is seen riding toward his sons off the side of the hill, he is sitting astride his horse, and his right hand is held up holding the reins; when his son Ben takes him off his horse, he is already dead.