When Ben and Clint enter the saloon then the livery stable, Clint's two-gun rig is hidden by his coat. When Clint follows Stark into the livery stable, his guns are strapped on the outside and over his coat. When he and Ben they ride out of town, Clint's guns are back under the coat.
When the shootout starts with the Jayhawkers, Ben fumbles pulling his gun out and it is aiming downwards when he starts shooting. The Jayhawkers are in front of him on horseback and he would have clearly missed them.
When Ben and Nella enter the cabin, they build a fire on the floor. It burns all night with no smoke in the room. They also enter at night, but the inside of the cabin is bathed in light despite them not lighting the fire or a lamp.
The scenes involving Nella's camp, the cabin, and non-establishing shots of the characters on horseback were filmed on a soundstage.
When the cavalry rides up to the cabin the morning after the snowstorm, they're riding in an already-existing path. But any path that had been there previously should have been filled in by the snow and blowing snow during the storm.
When Nella tries on the corset, she is already wearing one underneath and has been all along.
When Ben, Clint, and Stark start driving the herd to Montana, car tracks are visible in the sand in front of the herd.
When the Jayhawkers are collecting money from Ben, a truck moves along the road in the far distance behind him.
The film begins in 1866, but most of the firearms used are from later. Most pistols are Colt Single-Action Army with various barrel lengths, introduced in 1873. Most of the lever-action rifles are Winchester model 1873s. The only firearm that is period is the Remington 1866 derringer Ben takes from Stark in the stable.
When the wagons are lowered in the mountain range, there is a short shot of the movies crew Trucks below in the valley, especially an orange Truck cab and another truck.
Where did the pack mule come from? Ben and Clint did not have one at the beginning of the movie. When they left with Stark from Mineral Springs, they still did not have one. When they arrived at the cabin Ben stated that they stayed there night before last. There was still no pack mule in view and doubt that they would have left it there alone for that long. When all 3 left the cabin there was a pack mule being pulled by Ben which subsequently fell in the river.