Continuity mistake: When the doctors are trying to save the patient from being killed, Mel Brooks runs up a long flight of steps in the clock tower. But, during his climb, he saw that there was a step which was cracked, so he avoided it by jumping over it. When he landed on the step above it, he fell through. While he was dangling from a step, you can see that the one that he jumped over is no longer cracked. (01:27:10)
Continuity mistake: The opening scene shows the exterior of a 747, which has 2 large windshields meeting at a center post. The cockpit interior shot shows an arrangement of 3 windows, with a smaller window across the middle instead of a center post.
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.