This slip up can be seen at different times through out the entire movie but the first time it can be noticed is when Doc is following Yan (China girl) down the street with the flowers. If you look quickly right behind Doc there will be an American flag. But take a closer look...it has all 50 stars. This movie is supposed to take place in the mid to late 1800's so there should only be about 25-30 states at this time, definitely not all 50.