Factual error: Near the end of the film Butch is complaining about the living conditions they have to endure - jungles, swamps, snakes, night work - and Sundance sarcastically retorts "Bitch, bitch, bitch!" In 1908 the term meant just what it literally means: "Female dog." It did not adopt its current meaning of "complain" until much later. At the time the film is set - outside the context of "female dog" - it was considered to be a serious obscenity, and it would not have been used to describe something as ordinary as someone moaning about his living conditions.
Factual error: In the beginning, Sundance is accused of cheating at cards and the man accusing Sundance doesn't know who he is. To prove it, Sundance shoots the holster off the man and then the gun from his hand, and then shoots the gun around the room. All this takes 9 bullets. The gun is a revolver holding 6 bullets.
Answer: Sundance is being sarcastic. Butch is a know-it-all, and he's been frequently proven wrong throughout the film, usually to their detriment, but continues to think he's the "smart one" of the duo. Sundance is just ribbing him.