Other mistake: Just before Ladyfingers starts as the dealer in the big poker game, we are shown how skilled she is, with a close-up of her hands doing an expert shuffle of the deck. Lancey even tells her she hasn't lost her touch. But then she deals the third and fourth cards so badly that they flip in the air and are easily seen as a red deuce and a black deuce. It happens toward the bottom of the screen but slowing down the DVD makes it obvious.
Factual error: When running through the train yard, all the engines are diesel. The movie is set in the 20's or 30's. Diesels were not in use then, only steam engines.
Continuity mistake: In the final hand, the first time we see Clancy's hand, it is the 8-9-10-Q of Diamonds; when the Kid calls, the cards are (a) reversed in order - Q-10-9-8) and worse, (b) throughout the entire movie, they are playing with Hoyle playing cards - but the reversed hand as well as Clancy's JD are from a deck of Bicycle cards, made by the United States Playing Card Co. In a real game, Clancy's hand would be dead, as it contains cards from an entirely different deck.
Deliberate mistake: During the funeral march you can see that one of the buildings has a 1960s-style air conditioner on the side of the building.