Atticus is an excellent attorney who loves the law and practices it with honor. He is also a responsible father who loves his children and guides them with patience and understanding as they grow up. As a man of sound judgment and good heart, he makes few mistakes in the novel, but Atticus is a human being and not without error.
As a father, Atticus makes the mistake of underestimating the depth of Bob Ewell's rage and the danger he poses to Jem and Scout. He lets the children walk home alone after the pageant, exposing them to the attack by the drunken Ewell. The evil of attacking children is simply beyond Atticus' comprehension; he had never considered the possibility.
Another mistake, perhaps, is that he does not step in sooner to protect Jem and Scout from Aunt Alexandra's snobbish attitudes and occasional cruelty. He acts as a mediator between Scout and her aunt, not realizing the genuine distress Alexandra causes his daughter. When he finally understands Scout's confusion and pain as Alexandra pressures the children to essentially change their own identities, Atticus gives Scout permission to ignore her aunt.
In his role as a lawyer, Atticus is a man of few, if any mistakes. His defense of Tom Robinson is masterful and heartfelt. His neighbors in Maycomb accept that Atticus takes Tom's case, but they are appalled that he actually defends Tom vigorously. In retrospect, Atticus might have made a mistake in not preparing Tom for the outcome of his trial. He could have told Tom in advance that he most likely would be convicted, but that an appeal would follow and a conviction would not be the end for him. He could have explained the appeals process. Had he done so, Tom might have been more prepared for the guilty verdict and not felt so much despair in prison that he runs blindly for the prison fence and is shot to death.
Also, acting as Tom's lawyer, Atticus makes a mistake in going alone to the jail at night to protect Tom. Again, he underestimates an adversary, in this case a lynch mob. It does not occur to him that the sheriff could be called away under false pretenses, leaving him alone and unarmed. Only Scout's innocent conversation with Walter Cunningham's father prevents tragedy.