  Kitaev's toric code is arguably the most studied quantum code and is expected
to be implemented in future generations of quantum computers. The
renormalisation decoders introduced by Duclos-Cianci and Poulin exhibit one of
the best trade-offs between efficiency and speed, but one question that was
left open is how they handle worst-case or adversarial errors, i.e. what is the
order of magnitude of the smallest weight of an error pattern that will be
wrongly decoded. We initiate such a study involving a simple hard-decision and
deterministic version of a renormalisation decoder. We exhibit an uncorrectable
error pattern whose weight scales like $d^{1/2}$ and prove that the decoder
corrects all error patterns of weight less than $\frac{5}{6}
d^{\log_{2}(6/5)}$, where $d$ is the minimum distance of the toric code.
