Each letter pair below is formed by the same step, apart from one. Pick the odd one out: AE, DH, IM, KN.

In AE the positions are 1 and 5, in DH they are 4 and 8, and in IM they are 9 and 13, so in each pair the second letter stands 4 places beyond the first. In KN the positions are 11 and 14, a step of only 3; the true partner of K would have been O, standing 15th. The pair is out by a single place, which is exactly what makes it easy to pass over.