So all this talk about plagiarism reminded me of a piece of news I came across 3 years ago. A light novel was withdrawn from bookstores after they found many lines that were considered too similar to ones used in another series. I find it interesting that a) these lines managed to be found and pointed out, and b) the author felt the need to pull that many lines to use in his work.
On the other hand, I know of other works that also pull and use lines from other works, but they are recognized as parody works, and the lines exist solely for the purpose of making clever references and jokes.
I think that in this case it could definitely be argued that this was an episode of accidental plagiarism. If the lines were not exactly identical, I would not jump to conclude that they were plagiarized.
The comment above mine brings up an interesting point. IT could be very possible that the author was unaware that his material was strikingly similar to another work. It makes one wonder how many other cases of such an accident exist. I’d imagine that things like this happen regularly.