In the string of letters D, G, J, N, P, S one letter has been set down in the wrong place. Which letter is it?

The rule is a step of three places: D 4, G 7, J 10, then 13, then 16, then 19 — that is D, G, J, M, P, S. The fourth term given is N, whose place is 14, one too far along; the rest all fit. P and S, which come after the faulty term, sit exactly where the rule puts them and look wrong only if the count is carried on from N; G is the second term and is three places past D as it should be.