Something caught in the hose?

Randomly, a VACUUM on a database will fail with the message:

NOTICE: BlowawayRelationBuffers(emailevents, 284): block 335 is referenced
(pri
vate 0, last 0, global 2)
FATAL 1: VACUUM (vc_rpfheap): BlowawayRelationBuffers returned -2

After that, all subsequent VACUUMs do the same. The database seems
entirely usable, but the message won't go away unless I unload and rebuild
the database. What's going on?

--
-- Xof <xof@chanticleer.com>