Localized nonabelian anomalies at orbifold fixed points
  Orbifold compactifications of 10D heterotic strings do allow different sets of chiral fermions at different fixed points. Even if the effective 4D theory is anomaly free by including the bulk fermions, there arise abelian and nonabelian fixed point anomalies from these chiral fermions. As the underlying string theory is well defined these localized fixed point anomalies of the chiral fermions are to be cancelled by a variant of the Green-Schwarz mechanism. 
Florian Gmeiner, Jihn E. Kim, Hyun Min Lee, Hans Peter Nilles