My favourite Enigma message - as told to me by a tour guide at Bletchley Park - was thought to be a stream of indecipherable gibberish.

Until a WREN noticed that the message never contained the letter Z.

One weakness of that generation of Enigma was that it could not self-encode a letter. That is, A was never encrypted to A.

This had no letter Zs. A statistical improbability. Unless, so she reckoned, the entire message consisted of only the letter Z repeated.

Apparently, their best guess was that a bored soldier sent a stream of Zzzzzz to a friend. That was enough to crack that day's key.

Of course, every guide at Bletchley has a range of stories they tell credulous geeks. But it is a delightful tale of how OpSec is everything.