Jem broke this cardinal rule in Chapter 14 of To Kill a Mockingbird shortly after Dill was found hiding under Scout's bed after running away from home. Dill hoped to stay in hiding for a while, but Jem recognized that Dill's parents would be worried, so he went to the man that he trusted most when problems arose: his father, Atticus. Although Jem must have felt that he was betraying his young friend--"Dill's eyes flickered at Jem, and Jem looked at the floor"--he called for Atticus to come into the room. "Dill's face went white," and Scout "felt sick." By ratting out Dill, Jem had broken "the remaining code of our childhood." But Jem had made the right decision, and soon Atticus had smoothed over Dill's troubles.
In To Kill a Mockingbird, Jem and Scout are as close as two siblings can be. They confide their secrets to each other, and they know that the secret will be kept secret. As the novel progresses, we see that Jem and Scout are growing up and maturing, but their special bond remains intact. However, we also see that Jem is starting to see himself more as a man, and begins to pull away from Scout in some ways.
In chapter 14, Dill runs away from home and hides under Scout's bed. When she and Jem find him, they are perplexed as to why Dill is there. Scout, of course, is thrilled that her friend is there, but Jem feels like he has to let Atticus know that Dill has run away. Scout is furious that her brother would betray them and tell Atticus. Scout can not believe that Jem would betray her and Dill in this way, but (to Scout) Jem does the unthinkable.
Dill's eyes flickered at Jem, 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?"
Of course, Scout eventually forgives Jem for breaking this code, but at the time, Scout can't believe that Jem would do this. There is a bond between Jem and Scout that cannot be broken by him telling Atticus the truth. Their bond will lead to them facing the most dangerous night of their lives, and that bond will be there forever.