It Shouldn't Happen to a Dog (movie, 1946)

Leon suspects something between his wife (Dorothy Granger) - talk about the pot calling the kettle black - and the milkman, who are actually talking about getting rid of the dog. Leon hires a detective. An escaped convict enters the house, knocks out Leon and ties him up in a sheet. The milkman picks up the sheet thinking it is the dog. Mrs. Errol realizes the mistake just before Leon is dropped off the pier.