I think the length of the quote has a lot to do with whether it is plagiarism or not. If you write a short sentence, say about 6 words or less, and it turns out that someone else has written those exact 6 words, I don't think you can call it plagiarism, unless you're frequently "just by accident" using the exact words that appeared in a publication, that were written by that same person.
And what about unpublished writings, such as the things we post on forums like this one? Can you really call it plagiarism? Not legally, but following the descriptions in my first paragraph, maybe it is. It is certainly ethically wrong, if you deliberately try to make it appear like your own writing.