It was the fact that a letter could never encrypt to itself that was, perhaps, the single biggest fault of the Enigma. In routine messages, the position of expected or candidate plaintext could be inferred from a lack of correlation of any characters, and any candidate plugboard configurations that forced auto-mapping could be immediately discarded. (Rotor choices, particularly the right-hand wheel, could be determined statistically, leaving the ring settings and plugboard configuration as the tedious, time-consuming bit.)