Trivia: Katherine Ross, (Etta Place) was caught operating a camera, filming some footage of the arrival of the train carrying the "super posse". In the late 60s the US film business was strict, closed shop union (to a great extent it still is) and Ross operating a camera was against every rule there is. Several senior crew members demanded her dismissal from the film but producer John Foreman and Unit Production Manager Lloyd Anderson, aware of the fact that a lot of scenes with her in it would have to be reshot at absurd expense, argued for a compromise to which the union agreed - none of the footage she shot would be used (it wasn't) and she would be asked not to be on set while scenes in which she was not involved were shot. Her gender was totally irrelevant to the issue. This is confirmed in William Goldman's excellent memoir, "Which Lie Did I Tell?"
Trivia: In 1981, Robert Redford founded the Sundance Film Festival, which is named after his character the Sundance Kid.
Trivia: The turn-of-the-century-style film (which plays alongside the opening credits) was originally intended to appear in the bulk of the story. On Butch, Sundance and Etta's trip through New York, they view this film, which depicts Butch and Sundance's deaths. It upsets Etta so much, it contributes to her later decision to return home by herself. The segment had an annoyed Butch and Sundance watching the film from the back of the theater, whispering comments like, "We never did that". The change was made when it was decided to make the trip through New York into a musical interlude over still-photos. The main reason for the change: the studio had just finished work on a "period" set of a New York street (ca.1900) for the film Hello Dolly and did not want this expensive set appearing in a different film first.
Trivia: Dustin Hoffman was considered for the role of Butch Cassidy.
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.