vacuum full fails: Parent tuple was not found[7.2]

[Postgres 7.2, SuSE Linux 7.3 (2.4.10), Pentium-III, gcc-3.0.4]
vacuum full fails on pg_class:

VACUUM FULL VERBOSE ANALYZE pg_class;
NOTICE: --Relation pg_class--
NOTICE: Pages 147: Changed 0, reaped 134, Empty 0, New 0; Tup 2559: Vac 0, Keep/VTL 666/222, UnUsed 7051, MinLen 116, MaxLen 176; Re-using: Free/Avail. Space 833148/831052; EndEmpty/Avail. Pages 0/116.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
NOTICE: Index pg_class_oid_index: Pages 45; Tuples 2559: Deleted 0.
CPU 0.02s/0.00u sec elapsed 0.05 sec.
NOTICE: Index pg_class_relname_index: Pages 149; Tuples 2559: Deleted 0.
CPU 0.04s/0.00u sec elapsed 0.11 sec.
ERROR: Parent tuple was not found

What does this mean, and how can I fix it?

Vacuum without the "full" directive works OK.

--
I cannot think why the whole bed of the ocean is
not one solid mass of oysters, so prolific they seem. Ah,
I am wandering! Strange how the brain controls the brain!
-- Sherlock Holmes in "The Dying Detective"