In Harper Lee's To Kill a Mockingbird, we may be able to refer to the first incident occurring between Bob Ewell and Atticus as taking place in the courtroom. In Chapter 17, Atticus exposes both Sheriff Tate's and Bob Ewell's testimonies as being full of lies when he asks Mr. Ewell to write his name. Prior to this, Atticus confirms with both Tate and Ewell that on the night in question, Mayella's right eye was blackened. One problem with that testimony and pegging Tom Robinson as the culprit is that Tom Robinson is right-handed. When we face people, we face them as opposites; therefore, a right-handed person would only be able to reach and hit the left side of an opposing person's face. When Atticus has Ewell sign his name, we learn that Ewell is left-handed; only a left-handed person would be able to blacken the right eye of an opposing person. When Ewell starts figuring out his testimony has been exposed by Atticus as being full of holes, his response, as Scout narrates is to say before the court, "He didn't see what his being left-handed had to do with it, that he was a Christ-fearing man and Atticus Finch was taking advantage of him" (Ch. 17).
A second incident between Ewell and Atticus is recorded at the end of Chapter 22. One morning, as Atticus is walking down the street past the post office, Bob Ewell approaches him, spits in his face, and threatens Atticus, saying, as Scout narrates, "He'd get him if it took the rest of his life."
Scout narrates a third incident at the end of Chapter 26 when she narrates gossip she has learned about Ewell's response towards Tom Robinson's death. Based on the gossip, it wasn't enough for Ewell to be satisfied with Tom's death. Instead, Ewell longs for more death and expresses his desires in the form of a threat, saying that Tom's death "made one down and about two more to go." In other words, Ewell is saying that now that Tom is dead, there are only two remaining who should die. Whom he is referring to when he says "two" is a bit ambiguous; however, the reader can suppose that at least one is Atticus and wonder if the other one or even both of the "two" refer to the Finch children.