Dracula (1979)

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: Near the end of the movie just before Van Helsing drives a stake through Dracula's heart, Dracula leaps out of his coffin and impales Van Helsing with the stake. It's very clear that the person getting impaled is a stuntman with an 'Olivier' mask.

Nicki

Revealing mistake: In the final moments of the scene where Dracula has been confronted by Dr. Van Helsing and a cross, Dracula flees by transforming into a wolf and jumping through a window. Look closely as Dracula approaches the window and you can see the mask worn by the stuntman just before plunging through the glass.

Dr. Jack Seward: Count, some wine?
Count Dracula: No thank you, Doctor. I never drink wine.

Count Dracula: You are a wise man, Professor, for someone who has not lived even a single lifetime.
Prof. Abraham Van Helsing: You flatter me, Count.
Count Dracula: But not wise enough to return to Holland at once, now that you have learned what you have learned.
Prof. Abraham Van Helsing: I prefer to remain.

Count Dracula: Listen to them - the children of the night. What sad music they make.