Examples of foreshadowing in Chapter 23 of To Kill a Mockingbird:

• "I wish Bob Ewell wouldn't chew tobacco."-- Atticus's remark about Ewell's spitting tobacco in his face. This is the first sentence of the chapter.
• "I'd rather it be me than that houseful of children out there."-- Atticus' remark to Jem that he would rather have Bob threaten him than take it out on the Ewell children. This foreshadows the depths of Bob's hatred: that he was willing to kill children to make good his threat.
• When Atticus tells his sister that Bob had gotten it all out of his system, Alexandra replies, "I wouldn't be so sure of that." She worries that Ewell may try something else besides spitting and threats. 
• Jem's statement that he thinks he knows why Boo stays shut away--"... because he wants to stay inside"-- is realized after Bob's attack on the children. He decides to come outside when there is a necessity to save his young friends, but after Scout walks him home--and he knows they will be safe with Ewell dead--he retreats back into the home, and Scout never sees him again.