During the fist fight between Vance and the Sundance Kid, Sundance grabs a bottle behind the saloon counter. In the next shot, it appears to have been smashed, but we don't see him doing it.
When Vance and Sundance are fighting behind the bar in the saloon their shadows are cast under them by an overhead (studio) light.
Vance takes cover behind a buggy with straw all around it. This is the same buggy that the outlaw gang earlier pushed, covered in burning straw, towards a posse.
The sign over the Marshal's office was spelled "Marshall's". There is only one L in Marshal.
When Cheyenne knocks out the man in the bank with her gun, she uses the rod or barrel end, not the butt which would not have the same desired effect.
There are no lights in the saloon but strong shadows are cast and often in two directions.
Although Billy the Kid is depicted as being a gang member in 1889, he was actually killed in 1881. John Younger was killed in a shootout, and Bob Younger was dying of tuberculosis in prison in 1889.