  I identify a source of Lambda_{QCD}/m_Q corrections to the assumption of
quark-hadron duality in the application of heavy quark methods to inclusive
heavy quark semileptonic decays Q->q l nu_l. These corrections could
substantially affect the accuracy of such methods in practical applications and
in particular compromise their utility for the extraction of the
Cabibbo-Kobayashi-Maskawa matrix element V_{cb}.
