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
Directed by: John Badham
Starring: Donald Pleasence, Laurence Olivier, Frank Langella, Kate Nelligan
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.
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.