  Given the increasing availability of lattice data for (unquenched) QCD with
$N_f=2$, it is worth while to check whether the generated vacuum significantly
deviates from the quenched one. I discuss a specific attempt to do this on the
basis of topological susceptibility data gained at various sea-quark masses,
since for this observable detailed predictions are available. The upshot is
that either discretization effects in dynamical simulations are still
untolerably large or the vacuum structure in 2-flavour QCD substantially
deviates from that in the theory with 3 (or 2+1) light quarks.
