I was talking last night to a California street cop who has seen major demographic changes in his small town in the last thirty years. He's also learned to speak pretty fair Spanish—good enough for streetwork.
But when he was in the process of learning, he recalls arresting a monoglot immigrant armed with a knife, and shouting "Lavate su mano!" This puzzled the immigrant, who stared down at his hands, and the knife in them, with a puzzled look. The officer had meant to say "Levanta su mano" which means "Raise your hands."
The word he had used instead means "Wash your hands!" Eventually the man figured out what he meant. "Oh, the knife!" and put it down, possibly influenced by the non-verbal communication of the officer pointing a pistol at him. See Spanish (Or An Immigration Moratorium) For Police Officers,for more on this.