Other mistake: At or just after the point where Steve McQueen restrains the diner owner from running out the door - McQueen's wedding ring is visible on his left hand throughout one entire long-shot. (He was 27, despite playing a teenager, and had been married to Neile Adams for more than a year).Sionnach_Dhu
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.