While pursuing Frank Baines at night, Lefty catches up and makes excessive noise alerting Frank to his presence. Frank ominously cocks his pistol in the dark. A few moments later, the scene cuts to Frank but shows his pistol with the hammer down. A few additional moments later, the scene again cuts back to Frank again and the pistol is now shown as cocked.
In this movie timeline, taking place in a warm month in 1889, Montana is still a Territory (becoming a State in November 1889. The characters, especially the Governor, talk about Montana being a State.
The reloading tab of Lefty's Henry is always in the down position indicating that the rifle is unloaded. Lefty could not have threatened anyone at the time.
Although Lefty does not appear to be left-handed -- he shoots right handed, rides right-handed (holding the reins in his left hand so that his right hand is free), and wears his sidearm on the left side, butt forward, so that he can draw it with his right hand -- he could have gotten the nickname for some other characteristic or habit.
Early in the pursuit of Frank Baines, Lefty decides to track him through the night by himself. During the following montage from evening to nightfall, two condensation trails from commercial jetliners crisscross the evening sky.
45 minutes into the film, Frank Baines is holding lefty and company at gunpoint. The bullets in his revolver cylinder are bright green dummy rounds and visible on screen for an extended period.
At 21:47- There is lens flare on the right side of the screen.
The barn is a modern barn. As seen by the steel plate and bolt at the bottom of the edge of the big door, it has a concrete foundation, which is something that would never exist out in the middle of nowhere.
In the saloon, as Tom walks over to the spittoon, a customer in a barber chair is in the background. The barber chair is an early 1900's model chair as indicated by the porcelain base and frame. Barber chairs in 1889 had bases and frames made from wood.
Early in the movie, Mrs. Johnson is helping to brand a calf. The sound playing is that of a goat bleating, not a calf.