Deliberate mistake: As the Lotus drives off the pier and enters the water, the exposed underbody is that of a normal car. After it enters the sea, the underbody we see is completely sealed, which it needs to be to allow the car to submerge.
Deliberate mistake: After the hotel murder, Brophy makes a series of photographic enlargements to prove Dr. Thorndyke's innocence. However, instead of holding his magnifying glass directly in front of his face as he examines the photos, Brophy angles the magnifying glass far to his right so the movie audience can see the magnifying glass details, but he can't possibly see anything.