When Winship and Tart meet the staff in the parlor, Hilda already has Dr. Tart's business card in her bosom before he gives it to her.
When they were meeting with Phylis, the time gun went off in Inspector Winship's coat, putting a big bullet hole in the back of it. Later in the movie, the hole is gone.
The acid in the tea is supposedly strong enough to dissolve a teaspoon, yet Inspector Winship doesn't feel it when it spills over his fingers.
When Inspector Winship and Doctor Tart leave Morley Manner the tailgate of the station is down. Later, the tailgate is up.
When Tart and Winship spy on Phyllis through the eyes of the painting, they slide pieces of wood away from the openings. When they finish, the sliding piece is gone from Tart's position.
Dr. Tart uses pigeons to deliver messages back to Scotland Yard, and he attempts to send them a few times at night. But pigeons can't fly at night because they can't see in the dark.
The title of the movie is "The Private Eyes", yet Tart and Winship are employed by Scotland Yard so they are not "private" detectives.
When Winship and Tart pull up to the gas station they notice how expensive gas is - "10 cents a gallon" - but they are in England where they don't use dollars, or cents.
After Winship and Tart Find Justin dead, you can see that Tart's candle has a section cut out and a light bulb in it. This was done to better illuminated their faces. This same idea was used in Young Frankenstein.
Jock, whose tongue was cut out, has cotton in his mouth in the first scene to cause his garbled speech.
It is revealed that Jock's tongue was cut out after he tried to steal a ruby. However, Jock opens his mouth later on, and his tongue is clearly visible. Also, Jock's speech, while slurred and garbled, can be understood if one listens closely. Someone who had his tongue cut off wouldn't be able to form spoken words of any kind.
At the petrol station when the old man is filling the tank, the sound effect is correct. When he finishes filling the car up he is no longer squeezing the pump yet the sound effect is still running as if he were.
While the English do call pennies Pence, they do refer to them occasionally as cents.
When Winship and Tart are driving to/from the Morley Manor, modern vehicles are visible driving along in the background on Interstate 40, which runs through the Biltmore Estate (the filming location) in Asheville, North Carolina.
When Dr. Tart reads the line "By the way, you're standing in bull ca-ca," the dialogue is clearly dubbed in, and one can tell by watching his lips that Dr. Tart was saying "you're standing in bullsh..." (it's not clear whether he finished the swear or broke off).
When Winship and Tart first meet the staff, the boom is clearly in view when Tart starts his line, "Eeeny, meeny, miney mo... catch a killer by the..."
As Phyllis leads Winship and Tart up the staircase.