  Toninelli, Biroli, and Fisher recently introduced the knights model, a
correlated percolation system (Phys. Rev. Lett. 96, 035702 (2006)). They
claimed to prove that the critical point of this model was the same as that for
directed percolation, and then used this to show (assuming a conjecture about
directed percolation, described later in a more detailed proof) that this model
has a discontinuous phase transition with a diverging correlation length.
However, there is an error in their work, so that these results are unproven
for their model. Their proofs can, however, be modified to work for a similar
model.
