There’s an even better example from ENIGMA (since the lack of letter-to-itself coding is inherent in the basic design, and would require a whole different kind of machine to avoid). Simplifying, the ENIGMA had three different coding wheels, which could be swapped around in any order, making different code keys. For some parts of the German military, the SOP was to change them around each day, but forbid keeping the same wheel in the same place for consecutive days. That effort to try and throw off codebreakers actually helped them, but allowing the codebreakers to eliminate many of the possible wheel configurations.