Revealing mistake: In the jail scene, Gene Hackman is reading from a book. When he turns the book too far towards the camera you catch a glimpse of his lines on brighter white paper attached inside the book.
Revealing mistake: English Bob is shooting the pheasants from the train. The first one he hits falls out the sky in front of the camera and you can see a wire attached to the bird trailing off camera to the right of the screen. (Slow-mo helps but is not necessary).
Revealing mistake: After Muney kills Hackman's character and he is leaving town in the torrential downpour, a large circle of white light is illuminating the street in between the buildings. This light cannot possibly be from any natural or earthly source (e.g., it's raining heavily, so no moon; no electricity during this period in history). Without the artificial light, the scene would be completely dark and unviewable.
Answer: Not crazy, but maybe there's a bit of the Mandela Effect at play. There's no such line in any draft of the script, and it's not in any version of the film I've seen (including theatrical). In any case, Little Bill already knows that Munny is going to kill him, hence his line, "I'll see you in hell." If there was ever any doubt in his mind what was going to happen-as he lay there staring down the barrel of a shotgun, wielded by a man who'd just murdered a roomful of people-it's certainly gone by that point.
Will was pointing a Spencer at Bill, not a shotgun.