At one point the judge at McCall's trial is interrupted by a man claiming that the bullet that killed Wild Bill Hickok is lodged in his wrist. He wants to testify so that in the years to come there will be a record of his involvement in Hickok's death. This man is a real historical figure named Bill Massie. Massie was seated across from Hickock when he was killed, and the fatal bullet did in fact pass through Hickok's body and lodge in Massie's wrist. In the confusion Massie initially thought that Hickok had shot him. The bullet remained lodged in Massie's body until his death in 1910. He supplemented his income by giving paid lectures about his role in the incident.
The first trial of Jack McCall was actually held at the McDaniel's Theater, not the Gem.
While working on this episode, Timothy Olyphant (Seth Bullock) told writer/creator David Milch stories of his own childhood and getting in fights at school, and that even if he were winning, he would have tears streaming down his face. The next day, Milch wrote the scene of Seth in tears while accosting Jack McCall, who asks him why he's crying.
When E.B. Farnum is cleaning the floor of a guest room and talking to himself, he notes that his initials stand for "Eustace Baily." The real E.B. Farnum's initials stood for "Ethan Bennet."
Wild Bill and Calamity Jane weren't as close as the show portrays. They did arrive in Deadwood together, but they met not that long before hand.