getting "ERROR: tuple concurrently updated" during concurrent grant/revoke operations (9.2.13)

I was experiencing a problem where a script which was simultaneously
loading multiple database backups to private schemas was reliably
drawing the error, "ERROR: tuple concurrently updated" during
grant/revoke steps of the restore. (The problem isn't at all serious
because the impacted statement is irrelevant to this server.) If I
create a file,

breakit.sql:
GRANT ALL ON SCHEMA public TO PUBLIC;
REVOKE ALL ON SCHEMA public FROM PUBLIC;

and bench it, it immediately draws the error:
[merlin(at)mernix ~]# pgbench -n -c16 -T30 -f breakit.sql postgres
Client 4 aborted in state 0: ERROR: tuple concurrently updated
Client 2 aborted in state 0: ERROR: tuple concurrently updated
Client 0 aborted in state 0: ERROR: tuple concurrently updated
Client 1 aborted in state 0: ERROR: tuple concurrently updated
Client 10 aborted in state 0: ERROR: tuple concurrently updated
Client 5 aborted in state 0: ERROR: tuple concurrently updated
Client 7 aborted in state 0: ERROR: tuple concurrently updated
Client 12 aborted in state 0: ERROR: tuple concurrently updated
Client 14 aborted in state 0: ERROR: tuple concurrently updated
Client 3 aborted in state 0: ERROR: tuple concurrently updated
Client 11 aborted in state 0: ERROR: tuple concurrently updated
Client 9 aborted in state 0: ERROR: tuple concurrently updated
Client 13 aborted in state 0: ERROR: tuple concurrently updated
Client 15 aborted in state 0: ERROR: tuple concurrently updated
Client 8 aborted in state 0: ERROR: tuple concurrently updated

Also confirmed this on 9.4.

merlin