  We investigate unambiguities in the extraction of the $\bar d - \bar u$
asymmetry from semi-inclusive production of pions in DIS. The role of several
effects beyond the quark-parton model (QPM) which lead to $N_p^{\pi^+} \ne
N_n^{\pi^+}$ and $N_p^{\pi^-} \ne N_n^{\pi^-}$ and may therefore cloud such an
extraction is studied. The results are discussed in the context of the recent
HERMES data. We find that the interaction of the resolved photon with the
nucleon significantly modifies the observed $\bar d - \bar u$ asymmetry. The
exclusive elastic production of $\rho$ mesons plays a similar role for the
large-$z$ data sample. Our estimate shows a rather small effect of the
spectator mechanism. Nuclear effects in the deuteron also look potentially
important but are difficult to estimate quantitatively.
  Throughout the paper we make in addition several general remarks concerning
semi-inclusive and exclusive production of mesons.
