What happened when the previous Receiver in Training failed in The Giver?
When the previous Receiver in Training failed, all the memories that the Giver had already transmitted to her reverted back to the people. It was a terrible time for the community, because they had no idea how to deal with the onslaught of new, conflicting memories and feelings. After a time, the intensity of the memories gradually faded away, but until then, the people had to endure a period of unprecedented chaos and suffering.
Rosemary, the previous Receiver in Training, was a brave child, but she was sensitive, and was finally overwhelmed by the memories the Giver transferred to her. She understood her duty and insisted that the Giver not spare her, but in the end she could not handle the burden, and requested that she be released. To the end, she retained a quiet dignity and stoic courage, even requesting that she be allowed to inject herself at her own release. She could not live with the despair brought about by the memories, however, and so when she died, the memories returned to the people, who were completely unprepared for them. The Giver himself was overcome with grief at Rosemary's failure, and could not help them with the turmoil that suddenly beset them.
Rosemary had trained for only a short time, and the number of memories she had received was much less than those Jonas already has. If something were to happen to Jonas, and his memories should return to the people, the results could be devastating (Chapter 18).