The original coding was quite sloppy about handling the case where
XLogReadBuffer fails (because the page has since been deleted).  This
would result in either "bad buffer id: 0" or an Assert failure during
replay, if indeed the page were no longer there.  In a couple of places
it also neglected to check whether the change had already been applied,
which would probably result in corrupted index contents.  I believe that
bug #5703 is an instance of the first problem.  These issues could show up
without replication, but only if you were unfortunate enough to crash
between modification of a GIN index and the next checkpoint.

Back-patch to 8.2, which is as far back as GIN has WAL support.