A while ago I read Between Silk and Cyanide, the autobiography of one of the main actors of WW2 cryptographers, Leo Marks. He had a whole unit working on 'indecipherables', too-hard-to-decrypt messages where the field-agents either misspelled words or misused the encryption keys. You can imagine how often agents misspelled, having to do all the encryption manually under great lots of stress.

(There's an interesting story on Operation North Pole, where Marks suspected that a Dutch resistance group had been overtaken by the SS, and it was actually the SS sending the encrypted messages: all the encryption and spelling was suddenly perfect)