In two scenes (1st, when Charley & Boss first go along the main street; 2nd, when they leave at the end of the film) a pair of distinctive horses are seen harnessed (1st) to a wagon parked in the street, then (2nd) as the front pair of two pairs on their wagon. These horses (bay/chestnut with light mane, tail and feet) are Haflingers: this type, originating in Austria, did not appear in America until 1958.
The building under construction on main street is being built using modern framing techniques and is built from modern, dimensional lumber. Buildings were mostly "post & beam" type back then and "rough cut" lumber was used for building construction well into the 20th century.
Before the final gunfight, Boss and Charley are in the general store. Boss asks to buy some candy, and the shopkeeper offers him several varieties, including "Ju-Ju Bees." This candy was first made in 1920 by the Henry Heide Candy Company.
In the scene where Charlie sees the Doctor's sister thru the open door, you can see that she has shaved her underarms. This did not become a practice until 1915. This movie is taking place in the 1880's.
After Boss and Charley have the confrontation with Sheriff Poole in the café' they return to Doc Barlow's house to check on Button. When Sue opens the front door and lets them in a clock shows the time as 9:35. They spend at least an hour or more at the house but as they are leaving another clock in the background shows the time as 9:27. A loss of eight minutes.
In a scene where Boss and Charley are on the range in front of a river with a high bank on the other side in the background, first there are cattle on the bank behind them, then the cattle aren't there, then they are there, etc.
When Charlie drops the plank onto the sidewalk step to cross the flooded street it lands on the far right side of the step. When Boss crosses over and steps onto the step, the plank is on the far left side of the step.
After the initial gunfight when Boss and Percy are trying to stop Charley from shooting the injured bad guy, Percy is holding his rifle with one hand. When he steps beside Boss to stop Charley, and the camera angle changes, he is holding it in both hands.
Button's shots give Charley and Boss a chance to hide behind a horse trough. Baxter's shots hit the trough, splashing water on Boss's hat. When Boss emerges to rush Baxter with a shotgun, the hat is dry.
Charlie fires 16-17 rapid fires shots from one single-action, six-shot revolver - without reloading. In fact, during the first volley, he fires four random shots and then actually "fans" the revolver and fires ten additional shots into a bad-guy gunslinger in less than seven seconds, without changing weapons or re-loading. And then fires a few more rounds at still standing gun-men. Costner admitted in an interview for this film that he has always wanted to film a scene where he fans a six-gun way over the realistic amount of shots, and that this scene was indeed very enjoyable to make.
Boss talks about the Doc and Sue saying it paints a pretty picture. Charlie says "Yeah, I hear they're worth a thousand words." That saying was not coined until a 1911 newspaper article quoting newspaper editor Tess Flanders discussing journalism and publicity.