Double stepping
A quirk in Enigma that let the middle rotor move two steps in a row, making rotor positions less random than they looked.
Each rotor had a notch that drove the next rotor one step when it passed. The middle rotor also had its own pawl against the left rotor, and right after it had carried the middle rotor along, the middle rotor could move again on the following keypress. Two consecutive steps broke the pattern the designers expected, and the number of distinct rotor positions came out below 26³.
The flaw was usable. Codebreakers at Bletchley Park had to account for the rotors' behaviour in their methods, and double stepping was one reason Enigma was weaker than its makers believed. It is a good example of how a mechanical detail in a cipher machine can matter more than the size of its key space.