When the previous receiver in training (The Giver's daughter Rosemary) failed in her training, her memories were released back into the community. She could not give them back to the Giver, and in the book's world, the memories don't just die with her.
Instead, the memories dispersed (it seems like they just went up in the air and people got them in their brains) and were felt by all the people. This was really bad because all of a sudden all the people of the community had to feel things and remember things that they were totally not ready for.
The child Rosemary was chosen to be the receiver of the memories. She was the Giver's daughter. At first she was doing alright, but as the harder memories of pain entered her, she became too stressed to handle the memories. She asked to be released. The memories did not have time to be transferred to another person so they went back to the people. The book says it was an awful time for the people.
The Giver has given Rosemary loneliness and it was very hard for her. Rosemary left the Giver after a session and went to an elder and asked to be released.
"The community lost Rosemary at only five weeks, and it was a disaster for them."(144)
"But there were those few terrible memories, the ones that overwhelmed her. For awhile they overwhelmed the community. "(144)