ERROR: unexpected hash relation size:

... x should be y

I do get these messages one 2 of my servers running mostly on identical
data. The servers are unix-based and are running 8.1.8 under linux (Gentoo).
On one server the message appears only after vacuum-ing, on the other I
had some errors after inserting into the table too (about 12 inserts, no
more messages except while vacuuming since then)

The affected table has only one index based upon an integer.

The table definition is:

p_code character varying(10)
a_nr integer
ch_nr integer
and some other columns

the Index is a hash base upon a_nr. The table stores protocols for
batches with batch number ch_nr and order number a_nr and product
identifier code p_code. None of these columns are primary keys.

On the backup_server I got eg. errors while inserting
INSERT INTO d_kochmi (p_code,a_nr,ch_nr ....)
VALUES('613CL8D110',56117,1 ...)
batch number (column 3) varied from 1 to 9.

Should I do some more inverstigation ?
On one server i dropped and recreated the index
(CREATE INDEX idx_d_kochmi ON d_kochmi (a_nr) ) and the error while
running VACCUM was gone.

I'm wondering if the use of the hash index is disabled when such an
error occurs or if the index is used with probably wrong data ?

Regards
Gerhard