  Motivated by recent disagreements in the context of AdS/CFT, we study the
non-planar sector of the BMN correspondence. In particular, we reconsider the
energy shift of states with two stringy excitations in light-cone string field
theory and explicitly determine its complete perturbative contribution from the
impurity-conserving channel. Surprisingly, our result neither agrees with
earlier leading order computations, nor reproduces the gauge theory prediction.
More than that, it features half-integer powers of the effective gauge coupling
$\lambda'$ representing a qualitative difference to gauge theory. Based on
supersymmetry we argue that the above truncation is not suited for conclusive
tests of the BMN duality.
