This reminds me of an anecdote from "The Codebook" from Simon Singh (highly recommended book btw.). One of the reasons, why the British could crack the Enigma

This reminds me of an anecdote from "The Codebook" from Simon Singh (highly recommended book btw.). One of the reasons, why the British could crack the Enigma code, was that German officers introduced rules on how to use the system. For the Enigma machines they had to choose three out of five cylinders in different positions. The officers thought it would be more secure if they impose a rule "never use the same cylinder in the same position the next day".