  In the calculation of non-leptonic weak decay rates, a "mismatch" arises when
the QCD evolution of the relevant weak hamiltonian down to hadronic scales is
performed in unquenched QCD, but the hadronic matrix elements are then computed
in (partially) quenched lattice QCD. This mismatch arises because the
transformation properties of penguin operators under chiral symmetry change in
the transition from unquenched to (partially) quenched QCD. Here we discuss
QCD-penguin contributions to $\Delta S=1$ matrix elements, and show that new
low-energy constants contribute at leading order in chiral perturbation theory
in this case. In the partially quenched case (in which sea quarks are present),
these low-energy constants are related to electro-magnetic penguins, while in
the quenched case (with no sea quarks) no such relation exists. As a simple
example, we give explicit results for $K^+\to\pi^+$ and $K^0\to vacuum$ matrix
elements, and discuss the implications for lattice determinations of
$K\to\pi\pi$ amplitudes from these matrix elements.
