Operation North Pole

Englandspiel Nordpol, or Operation North Pole, was a successful Second World War counterintelligence operation conducted by Germany's military intelligence (the Abwehr) between 1942 and 1944.
On the night of 6-7 November 1941 two SOE agents, Huub Lauwers and Thys Taconis were parachuted into the Netherlands and dropped over Stegerveld, near Ommen. Lauwers was captured on 6 March 1942, whilst Taconis was captured 3 days later on 9 March. Lauwers was persuaded to send messages back to London by the Germans, in which he intentionally left out two security checks. This should have automatically sounded 'alarm bells' with those who received the messages, but for some inexplicable reason, it did not. Whether this was just a genuine mistake or something more sinister has never been fully ascertained. After all, security checks were in place to ensure that messages received from agents in the field were genuine and were part of the SOE's own transmission protocol.
As no one in London realised messages being received from SOE agents in the Netherlands were being sent under the control and direction of German military intelligence, more and more agents and equipment followed unabated for more than 18 months. Of the 54 SOE agents sent to the Netherlands from England during Operation North Pole, 50 died or were executed whilst being held prisoner by the Germans.