[ADMIN] VACUUM problem

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Hello.

Recently some users started to complain about:
Warning: PostgresSQL query failed: FATAL 1: btree: lost page in the chain of duplicates

I  noticed  that  VACUUM removes this problem, but no making
VACUUM I've got:
NOTICE:  BlowawayRelationBuffers(confread, 45): block 154 is referenced (private 0, 
last 0,
global 1)
FATAL 1:  VACUUM (vc_rpfheap): BlowawayRelationBuffers returned -2

What it can be? I'm using 6.3.2

Andrey