When Jack Langrishe goes to the bank to make a deposit and a withdrawal, he removes the bag from his shoulder as he approaches Alma. When the camera angle changes, he removes the bag from his shoulder again.
When General Fields wheels "Drunk Steve" into the No.10, he props his right hand on the table and slams down approximately 12 coins by that hand. When the camera angle changes, there are now only approximately 6 coins by his hand.
When Hugo Jarry says he will be Alcibiades to Hearst's Socrates, Hearst asks him if he seeks a "homosexual relationship." The term "homosexual" first appeared in English in 1892. Season 3 of the show is set in 1877.
Wu is shown drawing a picture, and it includes the characters for "Chinese People". The middle character he uses is in the simplified Chinese form that is used in modern (Communist) China, which was not adopted until the 1950s.
During the Amateur Night performance, a woman singing "Jesus Loves Me" is approached by a drunk holding a modern bottle of Bulleit Bourbon. Although this bourbon was initially produced in the 19th century, it ceased in 1860, prior to the time period of Deadwood. The bottle is the same design as that of its present incarnation, revived in 1999.