  At small lattice spacing, or when using overlap fermions, lattice QCD
simulations tend to become stuck in a single topological sector. Physical
observables, e.g.\ hadron masses, then differ from their full QCD counterparts
by $1/V$ corrections, where $V$ is the spacetime volume. These corrections can
be calculated order by order using the saddle point method. We calculate all
corrections proportional to $1/V^2$ and $1/V^3$ and test the resulting
equations for several models: a quantum mechanical particle on a circle, the
Schwinger model and SU(2) Yang-Mills theory.
