In the final scene there is a close shot of Grump's feet close together showing the shoes on the wrong feet. The next, longer, shot shows his feet some distance apart.
In the first scene when the detective is called, his hand picks up the phone and he says he's not interested in the case and hangs up. When it rings again, a woman's hand picks up the phone and gives the time. It is Ginger Rogers' voice, though she and the detective haven't met yet.
Phil Winston (Lyle Talbot) is wearing a wedding ring throughout the film, despite playing an unmarried man.
When the villain is in his secret room peeking at his victims, he wears a glossy dark robe and full hood, hiding his face and body. There is no need for the disguise as no one even knows the room exists, he does not leave the room to kill and no one will see him.