Continuity mistake: Early in the courtroom scene there is a shot of Gregory Peck that is reversed. The part in his hair is not on the same side of his head that it is in the rest of the film.
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.
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: At the beginning of the movie when Jem and Scout boost up Dill to see inside the courtroom, there is no way Dill would be able to reach such a high place with Jem and Scout's hands only going about waist high.
Continuity mistake: As Jem, Scout and Dill are walking up to the courthouse, they pass a group of men playing checkers. One of the men speaks to Jem and the three stop momentarily. As they respond to the man, two black women as extras pass by them, walking toward the street. The next shot is a continuation of the scene with overlapping dialogue, however, the two black women are nowhere to be seen and there is a white woman with a white hat where the two black women should have been. Jem, Scout and Dill pause on the handrail before running up the steps, and as the camera pans back, all three women are once again walking past the old men.
Continuity mistake: When the children go to the courthouse and up to the courtroom door, there are two solid doors with covered panels (that do not let light through), with a clear transom window above the doors. However, when the children boost Dill up to see into the courtroom, the doors' top panels are clear windows.
Continuity mistake: After Scout gets out of her ham costume, she runs down the street towards her house and as she runs past the street lamp, someone didn't do a very good job editing the film together.