What symbol in "The Catcher in the Rye" has to do with the theme loss of innocence?

As lizbv mentioned the profanity on the wall. Another example would be the Little Shirley beans record.

The Shirley Beans Record represents a world without phoniness (innocence). Holden mentions the fact that he hates phonies several times in the book, and when he first talks about the record he mentions the fact that he enjoys the singer’s voice, and how she doesn’t sing in an overly cutesy way, as some children would. The breaking of the record symbolizes the loss of innocence, him fumbling to put it back together is a symbol of him trying to save the innocence that once was.