Note that "police police police police" is a grammatically valid sentence, with multiple different parsings, one of which we could rephrase as "the people who keep a watchful eye on what the police are doing, keep a watchful eye on what the police are doing" -- that is, the police police are policing the police -- so it's even true.

(Cf. https://en.wikipedia.org/wiki/Buffalo_buffalo_Buffalo_buffal....)