The opening scene has a subtitle "Friday, May 21, 1878", but May 21, 1878, was a Tuesday.
Sam Bass was a notorious outlaw, not a lawman, so Mayor Polk hiring him as the new town marshal would be an act of madness.
Cane's "hotel room" is clearly a hallway; the door opens the wrong way and has the stain of a room number on the "inside."
When the brothel owner is speaking with the mayor, Wormy is supposed to enter through the door in the background. He blows his cue by entering too early, realizes this, and quickly slips back behind the door.
When Joshua and Cane fight in the Marshal's Office, they bump into the jail cell which shakes like a heavy steel cage shouldn't.
During the climactic gunfight, Beverly Garland's horse does not react to the shots that are being fired in its direction.
At the beginning after the sheriff is killed his wife runs out the door to shoot at the man riding away, it is obviously full daylight. When she comes back into the office the world behind outside the door is completely black, as though it was the middle of the night.
When Cane and Rose are in the tree, he remarks that they can see the Big Dipper, although it is clearly daytime.
In the woods when the letter was intercepted, the shadow cast by a very bright studio light, placed just to the left and behind the camera, is very visible when the horse and rider carrying the letter pass in front of the light.