Plagiarism is like patent law: You cannot patent something that is not original; you cannot patent "cake" or "bread". Only original mechanisms can be patented: The common screw has been around for so long, you cannot go patent it. You cannot patent the idea of a clock, or coffee cup, or book.
Similarly in plagiarism, you do not plagiarize somebody if you use a sequence of words that obviously (or failing that, you can show) is widely used elsewhere. "My country, right or wrong!" is not plagiarism, it was widely said in the 1870's.
Plagiarism is non-attributed use of original and unique wording. As a rough rule of thumb, the longer the sequence of words involved, when found in some other work, the more probable it becomes the combination was plagiarized. Thus it becomes difficult to find a sequence of, say, 10 words, appearing in works by multiple authors, that were NOT plagiarized. The same may be true (to a lesser extent as many other answers provide examples) for five words, I haven't heard of any five word rule.
I also would not make such a rule in my university, if I sat on a committee deciding such a thing. "Wherefore art thou, Romeo?" is four words and plagiarized. Elementary, dear Watson! is three and plagiarized, although these are both forgivable offenses.
Plagiarism is about stealing original art (word choices and construction) and/or original ideas, to pass them off as your own original construction. As a professor I do not require a reason or have to meet any standard to suspect a student (or other academic) of plagiarism and therefore search for bits I think were probably plagiarized. It is part of my job, as a teacher and academic, to not be so gullible and trusting.
To those that complain I should consider people innocent until proven guilty, that stricture only applies to punishments by those in authority: obviously police must investigate crimes and consider a person guilty in order to bring them to trial and prove it. Punishment should require proof, suspicion and a search for such proof do not require any proof.