Jeffery Jones's newspaperman character refers to himself as an "ink-stained wretch." This term for reporters wasn't coined until the early 20th century, either by Alexander Woollcott or by early sportswriter Trent Frayne.
When Bullock and Star are unloading their wagon, and the guy keeps yelling at them to hurry up, Bullock grabs the last bag off of the wagon. When the camera cuts back to the wagon, there are now several bags still unloaded.
In the opening scene as Bullock and Star are conversing about Byron Sampson's impending arrival, the prisoner in the cell behind them is leaning against the bars trying to regain Bullocks attention. Yet as the camera switches between the two protagonists, the prisoner's arms change position drastically between shots.
When Calamity Jane draws her revolver and orders Doc Cochran to wait for her, it is a Colt Peacemaker. As she puts the gun back in her holster, it is a Richards Mason conversion of a Colt cap and ball revolver.
The man who was shot through the head with a Derringer pistol in Al's hotel, clearly has an entry and exit wound above his eyebrows when the Doc is examining him. Afterwards, when the Doc "prods" the wounds, they have moved to the sides of his head, at his temples.
As Bullock is driving the wagon into Deadwood for the first time, the front of the wagon changes, depending on the camera angle. When the camera is behind Bullock looking down, the wagon has a tall square front that goes up as high as his knees. When the camera is in front, looking up from street level, the front is angled and lower and Bullock is resting his feet on it.
Seth Bullock states several times that he was born in Etobicoke, Ontario, when in fact he was born July 23, 1849 in Amherstburg, Upper Canada. Ontario is one of the four original provinces of Canada when the federal nation was created July 1, 1867, when the British North America Act, 1867 (or 'BNA Act') came into force
In the overhead shot of the street, just before Bullock and Hickcock's shootout with the guy on the horse, the section of street where the stunt man and horse are going to fall is visibly different from the street surface around it, apparently torn up by one or more previous takes.