What are some errors in judgment that Atticus makes in To Kill a Mockingbird?

If Atticus made an error in judgment, it was in not taking Bob Ewell seriously. Atticus believed Bob Ewell was all talk, not a real threat. 

Atticus did his best to defend Tom Robinson even though he knew convincing the jury Tom was innocent was next to impossible. He knew subjecting his children to the criticism and drama would be difficult, but he believed that it would be good for them to learn life lessons. Atticus did so well during the trial in defending Tom Robinson that the jury even deliberated, rather than immediately convicting him. 

Bob Ewell was humiliated by the trial. Atticus dragged his family’s history and dirty laundry into the open. Even though Robinson was convicted, Ewell was aware no one really believed Tom did anything to Mayella Ewell. Bob Ewell was exposed to all of Maycomb as a negligent, abusive drunkard.

Atticus did not really react to Bob Ewell spitting in his face.  He believed Ewell had said his piece, saved face, and would be able to move on. When Scout and Jem ask Atticus to carry a gun for protection, he tells his children he is not afraid of Ewell, saying,

I destroyed his last shred of credibility at that trial, if he had any to begin with. The man had to have some kind of comeback, his kind always does. So if spitting in my face and threatening me saved Mayella Ewell one extra beating, that’s something I’ll gladly take. He had to take it out on somebody and I’d rather it be me than that houseful of children out there (Chapter 23).

Atticus should have been a little more cautious, even if he really did believe that Ewell was done. Ewell was heard saying “it made one down and about two more to go” after Tom Robinson was killed going over the prison fence (Chapter 25). That sounds like a threat directed toward Atticus’s children. 

It probably would have been better if someone had walked Scout and Jem home on Halloween night. Atticus felt there was no danger because Scout and Jem were together and Maycomb was a safe town. The children had to walk through areas where there were not a lot of people around, however, and this made them vulnerable to Bob Ewell. Fortunately, Boo Radley was watching out for them and intervened.