One of the cars (License plate 263 with the silver bed springs sticking out the back) evacuating the Department of Agriculture camp site leaves the camp twice, once before the Joads pack up and once after.
When the Joads stop and ask for a loaf of bread and the waitress says they only have 15 cent loaves and the owner/cook tells her to let them have it she tells them to go ahead "Fred says it is okay". Later as the truck drivers leave leaving their change because she was so nice and sold 2 pieces of candy that cost 5 cents each for a penny when she saw the kids, she calls him Bert.
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.
When the Joads pull over to fix a flat on their truck they stop in a small depression that leans the truck to the left. In the very next shot the truck is in a different spot and leaning to the right.
The Joads' truck was actually a converted Hudson touring car. In many scenes, the Hudson Motor Car Company white triangle logo is seen at the top of the radiator. In other scenes, it is missing. The Hudson logo magically vanishes, then reappears during the entire movie.
When Casy and Tom are walking along the road towards the Joads' old farm, their shadows can be seen on the painted backdrop behind them. Also you can hear the reverberation caused by the film stage when they speak.
Beginning of film, Fonda and Caradine are approaching the abandoned Joad home. The front window appears to be lit from the inside. Perhaps a reflection of something offset, but very obvious.
As the Joads begin their sojourn west, they are shown starting on US Route 66 in Oklahoma. Immediate subsequent shots show signs for the city limits of both Sallisaw OK and Checotah OK. US Route 66 did not go through either of those cities, which are due east of Oklahoma City. US Route 66 carried travelers from Tulsa in the northeast. The Joads would have taken US Route 64 to US Route 266 to US Route 62 from Sallisaw to Oklahoma City, at which point they would have continued to California on US Route 66. Today, the Joads' entire route would align with Interstate 40 beginning in eastern Oklahoma.
The character, Noah (Frank Sully), after he's seen playing with his boat in the river, simply drops out of the story without any explanation, and does not appear again. In the book there is a brief reference to him going off on his own, but no explanation, whatever, is given in the film for his departure.
In the beginning when Tom is asking the truck driver for a lift, the driver states "Can't you see that sticker?" referring to the 'No Riders Allowed' sticker in his front driver side windshield. Tom states that he can see it, but he is standing beside the drivers side door and only approached the truck from behind throughout the scene. With the A pillar blocking his view he would not be able to see it.