Revealing mistake: There is a scene which features a nighttime view of Carfax Abbey, Dracula's home. This movie was set in the late 1800's/early 1900's, but if you look closely at the left-hand portion of the screen, you will see the lights of an airplane as it flies in the skies above the castle.
Revealing mistake: The British intelligence officer Wells, who is being trained to infiltrate the Gestapo headquarters at Lyon, would never pass as a German because of his slight but still quite noticable accent. (00:47:45)
Revealing mistake: When the boy accidentally hits the old man with the ball, he falls over. Note that you can see the stuntman's full head of hair as he hits the ground. When you return to the close-ups of the old man as he gets up, that extra hair is gone.
Revealing mistake: After the waitress brings George his fresh cup of hot coffee, he immediate takes a sip, and it spills down his chin from being induced by dental anesthesia, instead of actually burning him.