On a trip home to Wales, Niall MacGinnis was stopped and searched by police. He was arrested as a German spy when the police found a photo in his wallet of MacGinnis dressed in a German sailor's uniform, standing next to what appeared to be a U-boat. In fact, it was a publicity photo from MacGinnis' role in this movie. MacGinnis spent several days in jail before documents were sent from London verifying that he had been in the movie.
Producer and Director Michael Powell forgot that Newfoundland was a Crown Colony, and not a part of Canada, and when they moved the full-sized submarine model there, it was impounded by Customs and Excise, which demanded that import duty be paid. The matter was finally resolved when Powell appealed to the Governor of Newfoundland, citing their work for the war effort. Newfoundland finally became a Canadian province in 1949.
Raymond Massey, Leslie Howard and Sir Laurence Olivier agreed to work for half of their usual fee for the war effort.
Screenwriter Emeric Pressburger said, "Goebbels (Joseph Goebbels, Nazi Propaganda Minister) considered himself a master of propaganda, but I thought I'd show him a thing or two." Ironically, Pressburger's status in Britain, at the time, was as an enemy alien. He was born and raised in Hungary, which had allied itself with Nazi Germany. So when he returned from filming in Canada, he found himself imprisoned and threatened with deportation. Thanks to the intervention of Producer and Director Michael Powell and the Ministry of Information, this did not happen.
Peter Cushing had gone to Hollywood before the war, but things hadn't worked out too well, and he ended up doing a variety of odd jobs, which is how he came to be making props (not acting) on this movie. One day he had the job of making flags for model boats to be pushed around a map, and he made a lot of swastikas and laid them out in his digs. They were found by his landlady, who promptly called the police.