"page is not marked all-visible" warning in regression tests

I got this last night in a perfectly standard build of HEAD:

*** /home/tgl/pgsql/src/test/regress/expected/sanity_check.out	Thu Jan 12 14:06:14 2012
--- /home/tgl/pgsql/src/test/regress/results/sanity_check.out	Mon Jun  4 20:28:39 2012
***************
*** 1,4 ****
--- 1,5 ----
  VACUUM;
+ WARNING:  page is not marked all-visible but visibility map bit is set in relation "pg_db_role_setting" page 0
  --
  -- sanity check, if we don't have indices the test will take years to
  -- complete.  But skip TOAST relations (since they will have varying

======================================================================

I didn't manage to reproduce it in a few tries, and I don't recall
having seen the like reported from the buildfarm either.  So there's
some low-probability bug in there ...

			regards, tom lane