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.
Chosen answer: I think Little Bill takes offense to this sort of thing taking place in his town on his watch. He tells Mr. Beauchamp while he is writing his biography something like, "I do not like assassins, or men of low character."