In the establishing shot of Tesla's book (which shows Tesla's picture on one page and the text on the other) the text is in a fairly small font and the lines spread wide across the page. In the second close-up insert of the text itself, the font is considerably larger and the lines considerably narrower.
When child Nicki and her brother John go to sleep in separate beds, she's sleeping in the bed on the right side of the room, as she's attacked by the vampire shortly after. After hearing a scream a few moments later, the adults find her in the bed on the left side of the room.
The last "German" bomber shown in the final air raid appears to be fictional: a composite of a B-18 "Bolo" USAAF bomber (judging by its nose) and an inserted triple tail.
In many stories, a werewolf can only take wolf form during a full moon. However, no such requirement is ever stated in this movie. It appears that Andreas' transformations are dictated by Tesla and have nothing to do with a full moon.
Flipped shot: A Nazi bomber flying over London has a backwards swastika on it.
When the page of Tesla's book on Vampirism is shown at the beginning of the film, the text printed on the page consists of five brief paragraphs which are then repeated to fill out the page.
When Lady Jane speaks to Nicki Saunders the morning after Jane and Sir Frederick went to Tesla's hotel room, the vampire fang marks on her throat are no longer there.
Like the Fw 200, the XC-12 that appears in the film is not a real airplane; it's a miniature model.
The story is inconsistent as to whether Nicki Saunders is Dr. Saunders' granddaughter or daughter.
The opening of the film takes place in October 1918, and the bulk takes place in September 1940, according to the Regent Hall concert program. Yet Tesla refers to the previous conflict as "23" years ago.