In 1942, uncertain whether one of its spies had been replaced by a German impersonator, Britain’s Special Operations Executive hit on a clever plan: After a regular radio communication, the British radio operator signed off with HH, short for “Heil Hitler,” the standard farewell among Nazi operators. His counterpart, “Netball,” responded HH automatically, giving himself away.

They confirmed this at the next session:

Netball was several minutes late for his sked (not significant) and signalled ‘q r u’ (‘I have no traffic for London’). Howell replied ‘q t c’ (‘We have a message for you’), and proceeded to transmit it (the message warned Netball never to send less than 150 letters). Howell then signalled ‘HH’, and Netball immediately replied ‘HH’.

‘Right,’ Nick was heard to say to his companion, ‘that’s it then.’

(From Leo Marks, Between Silk and Cyanide, 2001.)