Cryptomnesia

Cryptomnesia occurs when a forgotten memory returns without its being recognised as such by the subject, who believes it is something new and original. It is a memory bias whereby a person may falsely recall generating a thought, an idea, a song, or a joke,[1] not deliberately engaging in plagiarism but rather experiencing a memory as if it were a new inspiration. Sentences in scientific papers that are identical to sentences from some of the references used to write the paper often stem from cryptomnesia.

http://en.wikipedia.org/wiki/Cryptomnesia