When Jonas is appointed Receiver-in-Training, the Chief Elder tells the community that this is a very special assignment. She explains that the current Receiver is very old and that the job will set Jonas apart from the community. The appointment of this new Receiver is very important, she says, because "We failed in our last selection" (61). She goes on to say that the experience was so bad that she can't even dwell on the matter because it caused "terrible discomfort." Jonas learns later while in training what happened. Rosemary, the precious receiver, had five weeks of training and asked for Release because it was too traumatizing for her. Apparently, the Giver gave too many difficult and painful memories to her too quickly in the training process and she couldn't take it. She asked for Release and they gave it to her.
Chapter 18 describes what happened to the community when the previous Receiver-in-Training asked for Release. The Giver explains the following to Jonas:
"I think I mentioned to you once. . . that when she was gone, the memories came back to the people. If you were to be lost in the river, Jonas, your memories would not be lost with you. Memories are forever.
"Rosemary had only those five weeks worth, and most of them were good ones. But there were those few terrible memories, the ones that had overwhelmed her. For a while they overwhelmed the community. All those feelings! They'd never experienced that before. . .
". . . if they lost you, with all the training you've had now, they'd have all those memories again themselves" (144).
Basically, when a Receiver dies, the memories are then set loose upon the citizens of the community, who are not prepared to deal with deep feelings or profound pain. The Giver admits that he had failed in his training because it forced Rosemary to want to be Released. As a result, there was a new rule placed on the Receiver-in-Training: he can't apply for Release.