Other mistake: In the opening scene on the airplane, one of the men tells Lt. Cable, "The captain wants to know if you'd like to sit up in the cockpit with him?" Buzz Adams, the "captain" referenced in this line, is only a lieutenant. Admittedly, "captain" is a standard aviation term for the pilot of an aircraft, but it would be unlikely to be used in this situation, since the term has a specific military meaning.
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.
Other mistake: During training, when the men are running and the two British men come out of the bushes then shoot at the ground, there are 3 rows of bullets going past but only 2 guns being fired.
Other mistake: Outlaws are chased by a mounted posse. One outlaw up on a hill shoots two of the posse members from long distance. Next shot, the two injured posse members are lying on stretchers and the chief deputy orders someone to go back to town and get a wagon. The stretchers were canvas, wooden handled, military style. Where did they come from?