Anomalies, D-flatness and small instantons

Recently, Witten has proposed a mechanism for symmetry enhancement in SO(32) heterotic string theory, where the singularity obtained by shrinking an instanton to zero size is resolved by the appearance of an Sp(1) gauge symmetry. In this short letter, we consider spacetime constraints from anomaly cancellation in six dimensions and D-flatness and demonstrate a subtlety which arises in the moduli space when many instantons are shrunk to zero size.