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.
Factual error: English Bob is shooting the pheasants from the train. The film is supposed to be set in 1881. Ring neck pheasants are not native to the US. They were imported from Shanghai, China and not stocked east of the Rockies until 1898.
Continuity mistake: In the bar scene where Gene Hackman confronts Clint Eastwood there is a whiskey bottle and a glass on the table in front of Eastwood. He shoves the glass forward and you definitely hear it chink against the bottle. In the next second there are several inches of space between the bottle and the glass.
Factual error: During the confrontation between Little Bill and English Bob outside the barber shop, all the Deputies cocked their guns - for emphasis. Then Mr. Beauchamp reaches into his bag to show that he only has books, the Deputies cock their guns again. With the rifles they have this would eject the shell that was in the barrel and cycle a new one. It doesn't.
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.
Other mistake: After the assassination in the outside toilet, as Clint and the kid are riding away they are very close together and being shot at by the other cowboys. The scene ends with the focus on two of the cowboys standing side by side but shooting in directions about 90 degrees apart. At the range they were shooting the targets would have to be 50 metres apart, but were actually last seen side by side.
Continuity mistake: When "The Schofield Kid" kills "Quick Mike" in the outhouse, all of the shots are to the chest. In the cutaway shot of the dead Quick Mike, he has a bullet hole in his forehead.