  We study pion-pion scattering in partially-quenched twisted-mass lattice QCD
using chiral perturbation theory. The specific partially-quenched setup
corresponds to that used in numerical lattice QCD calculations of the $I=0$
scattering length. We study the discretization errors proportional to $a^2$,
with $a$ the lattice spacing, and the errors that arise due to the use of
L\"uscher's two-particle quantization condition in a theory that is not
unitary. We argue that the former can be as large as $\sim 100\%$, but explain
how they can be systematically subtracted using a calculation of the $I=2$
scattering amplitude in the same partially-quenched framework. We estimate the
error from the violation of unitarity to be $\sim 25\%$, and argue that this
error will be difficult to reduce in practice.
