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:
Pages45; 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 isnot 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"