Atticus is a mockingbird figure. He fits the definition of one who only does good for others. In fact, Atticus might be the embodiment of this definition in the book. That said, this does not make him perfect. There are a few places where he is naive.
First, he is naive when he goes to protect Tom Robinson. He probably thought that nothing would have happened. But if Scout was not there, the mob may have killed him along with Tom. He underestimates the power of racism.
Second, he is also naive when he thought that Bob Ewell would stop at spitting in his face. After this incident, he did not think of Bob any more. He thought that that chapter of his life was over. He did not expect in the least that Bob Ewell would seek to attack his children.
Third, he is also naive when he wants to hold to the letter of the law instead of the spirit of the law at the end of the book. He should have known that it was more important to protect Boo and Jem. Like Heck said, he should "Let the dead bury the dead."