IIRC one cryptanalyst realised that the first 4 characters in ciphers from one particular operator never included the letter 'L'. They deduced that that operator always sent 4 Ls as an initial test, since L was on the right hand side of the keyboard and therefore the easiest key to hit for a right-hander. With LLLL as your known plaintext, decoding that operator's messages became a lot easier!
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IIRC one cryptanalyst realised that the first 4 characters in ciphers from one particular operator never included the letter 'L'. They deduced that that operator always sent 4 Ls as an initial test, since L was on the right hand side of the keyboard and therefore the easiest key to hit for a right-hander. With LLLL as your known plaintext, decoding that operator's messages became a lot easier!