Message key
The Enigma message key was three randomly chosen letters that set up each message, sent encrypted twice.
The operator chose three letters as the message key, typed them twice in a row, and encrypted that result with the Grundstellung. The message itself was then encrypted with the message key as its starting position. The repetition was meant to guard against transmission errors, but it gave cryptanalysts a structure to exploit.
In 1932, Marian Rejewski used permutation theory on the doubly encrypted keys and reconstructed the rotor wiring without having it. That insight underpinned the Polish bomba machine, and the British Bombe descends from it. The message key was a small technical gap with large consequences.