When the disguised Man in the Green Hat gives Quackie lumps (by means of a mallet), he gave him 2 whacks instead of 3, even though Quackie ended up with three lumps on his head.
Instead of the 1955-1964 theme, the 1946-1955 theme plays over the opening titles. [This should not be considered a Goof as it likely was intentional to provide the "ambiance" of an old(er)-time detective story.]