"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 -- --
sanitycheck, if we don't have indices the test will take years to -- complete.  But skip TOAST relations (since they
willhave varying
 

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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