Visible crew/equipment: When James Stewart tells Doris Day that he lied about the telephone call being from the hotel, Doris gets excited and James struggles with her on the bed. During that scene, you can see on the upper left corner a shadow of a microphone.
Revealing mistake: A couple cross the road to enter Victoria Coach Station, London. There is the noise of a coach braking hard and they throw themselves away from a coach just inside the entrance, which obviously was not moving at the time. They walk into the station and talk to an inspector, while the same coach is driven out of the entrance past where it is supposedly parked.
Continuity mistake: When we see Dorlock Holmes at his desk at the start of the cartoon, you can see that he is writing with the feather in his right hand, but in the next shot he is holding the feather with his left hand.