Continuity mistake: When Bob Ewell is on the witness stand and Atticus asks him if he ran for a doctor, Atticus is hovering over Ewell and his visible shadow behind Ewell reflects this. When the camera shifts to Atticus, he is a good 10-15 feet away. When the camera returns to Ewell, Atticus' shadow is still there.
Factual error: The time period of the film is 1932. In the opening title sequence, you see someone playing with Crayola Crayons (I assume it's Scout). One of the Crayons is labeled "Melon". Binney & Smith didn't sell the Melon colored crayon until 1949. Crayons produced from 1903 to 1948 sold for a nickel and contained only eight colors: red, orange, yellow, green, blue, violet, brown, and black.