backend dies suddenly after a lot of error messages

Hi,

We have problems with backend processes that close the channel because of
palloc() failures. When an INSERT statement fails, the backend reports an
error (e.g. `Cannot insert a duplicate key into a unique index') and
allocates a few bytes more memory. The next SQL statement that fails
causes the backend to allocate more memory again, etc. until we have no
more virtual memory left. Is this a bug?
We are using postgres 6.4.2 on FreeBSD 2.2.8.

It also works with psql:

toy=> create table mytable (i integer unique);
NOTICE:  CREATE TABLE/UNIQUE will create implicit index mytable_i_key for
table mytable
CREATE
toy=> \q

~ $ # now do a lot of inserts that cause error messages:
~ $ while true; do echo "INSERT INTO mytable VALUES (1);"; done | psql toy
INSERT INTO mytable VALUES (1);
ERROR:  Cannot insert a duplicate key into a unique index
...quite a lot of these messages
INSERT INTO mytable VALUES (1);
ERROR:  Cannot insert a duplicate key into a unique index
INSERT INTO mytable VALUES (1);

pqReadData() -- backend closed the channel unexpectedly.
        This probably means the backend terminated abnormally before or
while processing the request.
We have lost the connection to the backend, so further processing is
impossible.  Terminating.

Hmm, why does the backend allocate more and more memory with each failed
INSERT ?
Any clues?

Thanks,
Mirko