When Paul goes into town, he has a shiner by his left eye from Michael's punch. When Paul returns one or two days later at most, the shiner is gone.
In Duke's San Francisco brothel/casino, Scott Joplin's "Maple Leaf Rag" is heard playing in the background. Joplin, born in 1868, didn't write the classic piano piece until the late 1890s, several decades after the California gold rush in which the movie is set.
(at round 1h 14 mins) Michael tosses Duke a hammer to help with a carpentry job. The hammer is a ball peen hammer used for sheet metal work, not a claw hammer that is commonly used by carpenters.