Other mistake: After Van Helsing realises Dracula's coffin is in the cellar of Michael Gough's house, he hears a scream and runs to find the housekeeper Gerda overwrought and hysterical. He needs to find out where Dracula has just gone, and so he gives her a slap. It's hilarious how loud the sound of this is.
Continuity mistake: In the final sequence where Dracula's foot has been disintegrated by the rays of the sun, his foot is seen firmly attached again seconds later as Van Helsing confronts him with a makeshift cross. In the scene Cushing, with his back to the camera, steps forward quickly from the foreground to cover the area where Lee's foot is briefly seen as having not been disintegrated.