  We investigate the performance of a quantum error-correcting code when pushed
beyond its intended capacity to protect information against errors, presenting
formulae for the probability of failure when the errors affect more qudits than
that specified by the code's minimum distance. Such formulae provide a means to
rank different codes of the same minimum distance. We consider both error
detection and error correction, treating explicit examples in the case of
stabilizer codes constructed from qubits and encoding a single qubit.
