Another instance that foreshadowed Mr. Ewell's attack occured well before the evening of the incident. At one point after the trial, Ewell made the comment that there was one down and two to go. This let us know Ewell was out to get a couple other people. My assumption would be that he wanted to get after Atticus and Judge Taylor for both allowing his reputation to be further destroyed.
Also, in the chapter of the pageant, the first 4 pages refer to the darkness in like 10 different ways. It is symbolically suggesting that something bad is about to happen.
On the way to the pageant, Scout and Jem are frightened by Cecil Jacobs, who has been lurking around them and eventually jumps out to scare them. This scene affects the reader later because on Jem and Scout’s trip home from the pageant they hear noises in the dark and believe Cecil Jacobs is giving them the same treatment. However, we later find out it is indeed not Cecil Jacobs. It is Bob Ewell, who has come to attack the two as vengeance for Atticus making him look like a fool during the trial.