  Recently, Abedi, Dykaar and Afshordi claimed evidence for a repeating damped
echo signal following the binary black hole merger gravitational-wave events
recorded in the first observational period of the Advanced LIGO
interferometers. We discuss the methods of data analysis and significance
estimation leading to this claim, and identify several important shortcomings.
We conclude that their analysis does not provide significant observational
evidence for the existence of Planck-scale structure at black hole horizons,
and suggest renewed analysis correcting for these shortcomings.
