Personally, I don't believe in anything called unintentional plagiarism.
There's one case that comes into my mind: people telling "I've read somewhere that <this> and <that>".
Is that (unintentional) plagiarism or not? Is the source important if only the general idea is passed?
I'd say that there's no general answer for this and it depends...
So I'd go by the same statement as said just before me:
Its case to case basis actually.