Revealing mistakes: When the Joads set out from the gas station to cross the desert, you see them pull away from the station twice.
Revealing mistakes: Tom Joad's semi-retarded brother, Noah, vanishes after the swimming-in-the-river sequence. In the book, Noah believes he's a burden on the family and runs away. In the film, no explanation is given for his disappearance.
Continuity: When Pa Joad and Tom are talking about how they got some money to go on the trip, poor Uncle John carries the bed spring out the door three times.