  Lattice QCD simulations tend to become stuck in a single topological sector
at fine lattice spacing or when using chirally symmetric overlap quarks. In
such cases physical observables differ from their full QCD counterparts by
finite volume corrections. These systematic errors need to be understood on a
quantitative level and possibly be removed. In this paper we extend an existing
relation from the literature between two-point correlation functions at fixed
and the corresponding hadron masses at unfixed topology by calculating all
terms proportional to $1/V^2$ and $1/V^3$, where $V$ is the spacetime volume.
Since parity is not a symmetry at fixed topology, parity mixing is
comprehensively discussed. In the second part of this work we apply our
equations to a simple model, quantum mechanics on a circle both for a free
particle and for a square-well potential, where we demonstrate in detail, how
to extract physically meaningful masses from computations or simulations at
fixed topology.
