via the Wiki
Jung gives the following example in Man and His Symbols.Friedrich Nietzsche’s book Thus Spoke Zarathustra
includes an almost word for word account of an incident also included
in a book published about 1835, half a century before Nietzsche wrote.
This is considered to be neither purposeful plagiarism nor pure
coincidence: Nietzsche’s sister confirmed that he had indeed read the
original account when he was 11 years old; and Nietzsche’s youthful
intellectual prowess, his later cognitive degeneration due to neurosyphilis, and his accompanying psychological deterioration (specifically, his increasing grandiosity
as manifested in his later behavior and writings) together strengthen
the likelihood that he happened to commit the passage to memory upon
initially reading it and later, after having lost his memory of
encountering it, assumed that his own mind had created it.