The Cornishmen at the Gem are supposedly speaking Cornish (for which they are made fun of). While Bullock and other characters state this, the Cornishmen are, in fact, speaking Irish Gaelic (Gaeilge). To add to the anachronism list, the last known monoglot Cornish speaker died in 1777, well before Deadwood 1876.
It's implied the Cornishmen don't speak English well enough to be understood. The last known monoglot Cornish speaker died in 1777, well before Deadwood was founded.
Steve Fields (Michael Harney) references kiting checks, a term that did not come into use until the 1920s. The practice was referenced as "flying a kite" in the 19th century.
The children seen writing on small slates are using pencils. Slates require chalk;
pencil marks on a slate would be difficult to read and would not be erasable.