"You dirty rat"

Movie gangster par excellence Jimmy Cagney is famous for saying, "You dirty rat," and spawning as many impersonators as there are rats in New York. Only Cagney never said that. Depending on whom you believe, it's a slimmed down version of "You dirty, double-crossing rat" in "Blonde Crazy" (1931) or "Take that, you dirty yellow rat!" in "Taxi!" (1932).

...misquote perhaps?