A writer produces a passage that closely matches, almost word for word, a paragraph she read years earlier in someone else's book, sincerely believing the passage to be entirely her own original creation, with no awareness that she is reproducing something she previously encountered. What is this?

This is cryptamnesia: the experience of not remembering that one is remembering, in which material genuinely encountered before is reproduced with the sincere belief that it is original, with no awareness that it is actually a memory. It is not confabulation, in which memory content, while sometimes elaborated with genuine conviction, is typically invented to fill a genuine gap rather than being actually previously encountered material. It is not ordinary plagiarism, which involves conscious intent to deceive, absent here since she genuinely believes the material is her own. And it is not a flashbulb memory, which concerns an unusually vivid, consciously recalled recollection, not an unwitting reproduction of previously encountered material believed to be new.