Intermittent "cache lookup failed for type" buildfarm failures

There is something rotten in the state of Denmark. Here are four recent
runs that failed with unexpected "cache lookup failed for type nnnn"
errors:

http://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=grouse&dt=2016-08-16%2008%3A39%3A03
http://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=nudibranch&dt=2016-08-13%2009%3A55%3A09
http://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=sungazer&dt=2016-08-09%2001%3A46%3A17
http://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=tern&dt=2016-08-09%2000%3A44%3A18

The first two are on HEAD, the second two on 9.5, which seems to rule out
my first thought that this has something to do with parallel query. It's
notable though that all the failing machines are PPC or S/390 ... maybe
big-endian related?

I grepped through the buildfarm logs and determined that there are exactly
zero similar failures going back as far as 2016-04-01. Now that we've had
four in a week, it seems certain that this indicates a bug introduced at
most a few days before Aug 9. A quick trawl through the git logs finds
no obvious candidates, though.

Any ideas?

regards, tom lane