In Harper Lee's To Kill a Mockingbird, in Chapter Seven Jem is upset because the knothole in the tree at the Radley's place has been cemented up.
The gifts that had been left in the tree for Jem and Scout had been something rare and special to Jem and his sister. Since they did not walk past the tree in the summer, they spent their months free from school without the excitement of finding these wonderful gifts. When school begins and they start to travel once more daily past the tree, the gifts unexpectedly start again and Jem and Scout are delighted.
While it is unclear that it is Boo Radley leaving the gifts, the kids are certain that they have special meaning to the owner—whoever that may be. This makes the surprises all the more valuable in the eyes of the Finch children—that someone is being so kind to them. They are appreciative of this honor: for they clearly understand what it means to share a treasured possession with someone else in the name of friendship.
When the knothole is cemented up by Nathan Radley, Jem perceives that the gesture was one of malice. Once again, nothing indicates that Jem knows the identity of the generous giver, but Nathan Radley's intent seems obvious to Jem—either it was meant to hurt their secretive benefactor, or the Finch children, or both. This brings to mind the idea of not killing a mockingbird—never harming something that does nothing but good.
While Scout tries to get Jem to come in that evening, he decides to wait outside for a bit instead:
He stood there until nightfall, and I waited for him. When we went in the house I saw he had been crying; his face was not dirty in the right places, but I thought it odd that I had not heard him.
There are two things that this event foreshadows. It hints at Jem's coming of age, when he begins to learn that evil, fear and cowardice lurk in the hearts of men—something that in his childhood he never before realized and therefore, was never concerned with.
In that Tom Robinson is an example of a mockingbird in the story, Jem's tears may foreshadow another event when the innocent are harmed despite their kindness—despite the fact that they cause no one else harm. In Chapter Twenty-two, after Tom is found guilty—even while his innocence is obvious to everyone—Jem cries unabashedly as Atticus, Scout and he walk home.
His face was streaked with angry tears as we made our way through the cheerful crowd. "It ain't right," he muttered. [...] "It ain't right, Atticus..."
"No, son, it's not right." [...]
"Atticus—" said Jem bleakly.
He turned in the doorway. "What, son?"
"How could they do it? How could they?"
This question could easily echo Jem's internal battle in Chapter Seven as to how Nathan Radley could have, for no good reason, cemented up the tree's knot-hole—the nest where their unknown admirer left his gifts. In both cases, Jem struggles to understand what could make a person be so unkind and so unfair toward someone that had done nothing wrong and did not deserve such treatment.