Deliberate mistake: Despite it being demonstrated multiple times earlier in the film that vampires do not have a reflection, a key moment features the undead Mina being revealed in a reflection in water. The filmmakers said they were aware of the discrepancy but ultimately decided the visual was striking enough to bend the rules for.
Dracula (1979)
1 deliberate mistake - chronological order
Directed by: John Badham
Starring: Donald Pleasence, Laurence Olivier, Frank Langella, Kate Nelligan
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.
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.