Dunkirk. The British lost two encryption machines on the beaches of Dunkirk. The Germans found them. They compared them with their own three-disk Enigma, which they deemed impossible to decrypt. Since the British machine had five disks, they considered it even more impossible, so they didn’t even try. So, despite the terrible mistake of leaving behind two encryption machines, the British actually gained a huge advantage. If the Germans hadn’t known about the five disks, they probably would have tried to decrypt it, and perhaps even succeeded, with tricks similar to those used by the British at Bletchley Park.
