Bletchley Park is the spiritual home of the computer and the workplace of the genius mathematician Alan Turing. However, it is perhaps best known for the code-breakers who cracked the German Enigma cipher (amongst others, some even more complex) during the second World War, seen as a key contribution to the allied war victory. The whole park was shrouded in secrecy for fear of secrets being exposed to opposition forces.
A number of years later, an old couple who had lived nearby all of their lives were visiting the (now revealed) site as tourists. They were watching a bungled demonstration of the Enigma machine by a tour guide who missed a key step. The old lady got up and said something like "that's not how you do it" before taking over the demonstration herself, showing the correct way to operate the machine.
The man, amazed, asked her if she worked at Bletchley when she was younger. "Yes" she replied, "I worked in this hut over here".
"Oh!" said her husband. "I worked in that hut over there!"
They'd met on the train, travelling home from work and due to the Official Secrets Act, had never confessed their real work to each other.