  The effects of spectator light quark in decays of heavy mesons are
considered, which vanish in the limit of factorization of matrix elements of
four-quark operators over the mesons. These effects include the difference of
the total widths as well as of the semileptonic decay rates between the D^0 and
D_s mesons and also a contribution to inclusive semileptonic decay rates of B^0
and B^\pm into the channel X_u l nu related to determination of the weak mixing
parameter V_{ub}. If the observed difference of the lifetimes between D_s and
D^0 mesons is attributed to non-factorizable terms, such terms can naturally
give rise to a significant difference in inclusive semileptonic decay rates of
these mesons, and to a light-flavor dependent contribution to decays B -> X_u l
nu. The latter contribution affects mostly the upper part of the inclusive
spectrum of the invariant mass of the lepton pair, and may significantly exceed
the previously claimed in the literature theoretical uncertainty in
determination of |V_{ub}|^2 from that part of the spectrum.
