Bext^2(G,T) can be nontrivial, even assuming GCH
  Using the consistency of some large cardinals we produce a model of Set Theory in which the generalized continuum hypothesis holds and for some torsion-free abelian group G of cardinality aleph_{omega +1} and for some torsion group T, Bext^2(G,T) not =0 . 
Menachem Magidor, Saharon Shelah