  We consider effects resulting from the use of different discretizations for
the valence and the sea quarks, considering Wilson and/or Ginsparg--Wilson
fermions. We assume that such effects appear through scaling violations that
can be studied using effective-lagrangian techniques. We show that a double
pole is present in flavor-neutral Goldstone meson propagators, even if the
flavor non-diagonal Goldstone mesons made out of valence or sea quark have
equal masses. We then consider some observables known to be anomalously
sensitive to the presence of a double pole. We find that the double-pole
enhanced scaling violations may turn out to be rather small in practice.
