Revealing mistake: In the opening sequence when Sundance shoots the gun belt off the card player, the film was cut to make the quick draw appear faster. You can see Butch's image jump across the screen in the background.
Revealing mistake: In the scene where Strother Martin is leading Butch and Sundance back with the payroll Strother is shot dead. While he is on the ground Butch and Sundance scramble down near his body. You can see Strother's eye twitching from the kicked up dust.
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.