  Schietgat, Ramon and Bruynooghe proposed a polynomial-time algorithm for
computing a maximum common subgraph under the block-and-bridge preserving
subgraph isomorphism (BBP-MCS) for outerplanar graphs. We show that the article
contains the following errors: (i) The running time of the presented approach
is claimed to be $\mathcal{O}(n^{2.5})$ for two graphs of order $n$. We show
that the algorithm of the authors allows no better bound than
$\mathcal{O}(n^4)$ when using state-of-the-art general purpose methods to solve
the matching instances arising as subproblems. This is even true for the
special case, where both input graphs are trees. (ii) The article suggests that
the dissimilarity measure derived from BBP-MCS is a metric. We show that the
triangle inequality is not always satisfied and, hence, it is not a metric.
Therefore, the dissimilarity measure should not be used in combination with
techniques that rely on or exploit the triangle inequality in any way. Where
possible, we give hints on techniques that are suitable to improve the
algorithm.
