Humour through syntactic ambiguity: the sentence depicted is intended to mean "this area is under video surveillance by the police to prevent crimes", but also means "this area is under video surveillance to prevent crimes committed by the police".
From a series of woodcuts dated to 1545, usually referred to in German as the Papstspotbilder or Papstspottbilder, and in English as Depictions of the Papacy, by Lucas Cranach, commissioned by Martin Luther. Title: Kissing the Pope's Feet. German peasants respond to a papal bull of Pope Paul III. Caption reads: "Don't frighten us Pope, with your ban, and don't be such a furious man. Otherwise we shall turn around and show you our rears."