Dill’s eyes flickered at Jem, and Jem looked at the floor. Then he rose and broke the remaining code of our childhood. He went out of the room and down the hall. “Atticus,” his voice was distant, “can you come here a minute, sir?”
Beneath its sweat-streaked dirt Dill’s face went white. I felt sick.
…Jem was standing in a corner of the room, looking like the traitor he was. “Dill, I had to tell him,” he said. “You can’t run three hundred miles off without your mother knowin’.”
We left him without a word.

– Harper Lee

To Kill a Mockingbird, Chapter 14. Jem breaks the last remaining code of childhood – not to tell on each other to adults – when he tells Atticus about Dill running away from home. But Jem knew that Dill’s parents would be worried, so he decides it is for the best to break the code.