Factual error: Sir Francis Walsingham was only a year older than Elizabeth.
Factual error: The supposedly Great War vintage gas mask James Whale finds and later gets Clayton to wear is in fact a British style introduced in the 1930's, and the specific variation shown was not made except during WW2. In addition, during the flashbacks to WW1, various pieces of WW2 equipment crop up (e.g. helmets with spring-loaded canvas covered chin straps instead of leather chin straps, and WW2 British pattern gas mask bags).
Factual error: A marriage at Marylebone Register Office is introduced by a shot of a brass plate which had presumably been made-up as a prop and which carries the incorrect wording 'Marylebone Registry Office'.