Edmund's room contains an iron maiden, invented in 1793 for display in museums.
England and Scotland were two separate kingdoms in 1485. There was no personal union between them until 1603, when king James VI of Scotland also inherited the English throne as James I.
When the cannon explodes at the end, and Prince Harry thinks it is the drains, he says he will get his plunger, but these were not invented until 1874 in New York City.
In the hunting scene McAngus tries but fails to sheath his dagger after using it to skin an animal, but instead holds it in place until his back is turned. With The Black Adder hanging upside-down behind him a reshoot was presumably not thought desirable.
The credits call Edmund's mother "Gertrude, Queen of Flanders". However, Flanders never had a queen, since it was never a kingdom. There was a medieval and early-modern county of Flanders, existing from 862 up until 1795. Queen Gertrude's accent does not sound Flemish either. Like Richard XII of Scotland, this may be a holdover from an earlier script draft.
Edmund refers to some letters as "slanderous." He should have said libelous, as slander is verbal.