A book by Daniel L. Schacter describes an instance of supposedly unintentional plagiarism by Friedrich Nietzsche in Thus Spake Zarathustra. One of Nietzsche's characters goes on a dreamlike journey that closely mimics a classic german fable. Presumably, Nietzsche heard this fable before, but forgot it until he wrote what he thought was a novel story.
I read a newspaper article recently detailing a similar story about Helen Keller. In her autobiography "The Story of My Life," Ms. Keller describes how, at age 12, she wrote a story � "The Frost King" � that created her own publishing scandal. "Mr. Anagnos was delighted with 'The Frost King,' and published it in one of the Perkins Institution reports," Ms. Keller wrote (Chapter 14 at afb.org/mylife). "This was the pinnacle of my happiness, from which I was in a little while dashed to earth. I had been in Boston only a short time when it was discovered that a story similar to 'The Frost King,' called 'The Frost Fairies' by Miss Margaret T. Canby, had appeared before I was born in a book called 'Birdie and His Friends.' The two stories were so much alike in thought and language that it was evident Miss Canby's story had been read to me, and that mine was � a plagiarism."