  In an influential recent paper, Harvey et al (2015) derive an upper limit to
the self-interaction cross section of dark matter ($\sigma_{\rm DM} < 0.47$
cm$^2$/g at 95\% confidence) by averaging the dark matter-galaxy offsets in a
sample of merging galaxy clusters. Using much more comprehensive data on the
same clusters, we identify several substantial errors in their offset
measurements. Correcting these errors relaxes the upper limit on $\sigma_{\rm
DM}$ to $\lesssim 2$ cm$^2$/g, following the Harvey et al prescription for
relating offsets to cross sections in a simple solid body scattering model.
Furthermore, many clusters in the sample violate the assumptions behind this
prescription, so even this revised upper limit should be used with caution.
Although this particular sample does not tightly constrain self-interacting
dark matter models when analyzed this way, we discuss how merger ensembles may
be used more effectively in the future. We conclude that errors inherent in
using single-band imaging to identify mass and light peaks do not necessarily
average out in a sample of this size, particularly when a handful of
substructures constitute a majority of the weight in the ensemble.
