When Capt. Hardt sits down to study the map provided by Lt. Ashington; there is a book lying on top of the map. But on the closeup of Capt. Hardt studying the map; the book has now changed orientation.
When the plotters arrive at the cottage to change over the teachers, the car pulls in, but when they go to dump the body, the car is facing the opposite direction.
When Captain Hardt looks down at the British fleet from the house, the view that is shown from his binoculars is actually one from sea level, not the view one would expect from a high vantage point.
When Captain Hardt is ordered by Tiel to pick up his motorcycle and go to bed; he is wearing his overcoat. But on the next cut as he is carrying the bike up the stairs toward his bedroom; he is no longer wearing the overcoat.
In the German submarine, the officers refer to depths in feet, and the depth gauge is calibrated in feet. On a German ship, depths would be measured in meters.
There has never been a HMS Connaught in the British Navy.
Ashington is constantly referred to as Commander but he wears the two rings of a Lieutenant on his sleeve. This may be because he is impersonating a traitor who was demoted to Lieutenant.
Identified in the credits and on her passport signature as "Anne Burnett", the newspaper report of the teachers' Orkneys appointment misnames her as "Ann Burnett", though this passes entirely without comment.
Early in the film the sniper is wearing a 1930s German helmet, not a 1910s helmet.
During the closeup of the kidnapping on the train about 15 minutes into the movie, the rapidly passing scenery is clearly being projected onto a screen as the screen edges are visible, and the peoples' shadows are also projected onto the screen.