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.
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.
Deliberate mistake: When The Kid visits the farm, the red truck in the background is a Fleetside truck that is definitely not from the 1930s.
Deliberate mistake: During the funeral march you can see that one of the guys dancing has 1960s-style sunglasses on.
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.
Revealing mistake: When Steve McQueen is in the hotel room very upset with Karl Malden because he was helping him win, he picks him up and slams him against the door. If you watch when this happens the entire wall shakes.
Continuity mistake: When The Kid comes in from walking in the rain, his clothes are soaked but his face and hair are bone dry.
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.
