BUG #5929: ERROR: found toasted toast chunk for toast value 260340218 in pg_toast_260339342

The following bug has been logged online:

Bug reference:      5929
Logged by:          Tambet Matiisen
Email address:      tambet.matiisen@gmail.com
PostgreSQL version: 8.4.7
Operating system:   PostgreSQL 8.4.7 on i486-pc-linux-gnu, compiled by GCC
gcc-4.4.real (Debian 4.4.5-10) 4.4.5, 32-bit
Description:        ERROR: found toasted toast chunk for toast value
260340218 in pg_toast_260339342
Details:

For a few days I've been getting this error from my nightly backup script:

Warning: pg_dump: SQL command failed pg_dump: Error message from server:
ERROR: found toasted toast chunk for toast value 260340218 in
pg_toast_260339342 pg_dump: The command was: COPY public.yhistud_urlcache
(id, url, params, sess_id, content) TO stdout; pg_dumpall: pg_dump failed on
database "yhistud", exiting
Warning: Failed to dump pgsql cluster

Yesterday I upgraded the server from 8.4.5 to 8.4.7, hoping that this error
will go away, but no success.

I've been getting occasional errors from backup script for several months,
but this time the error is recurring. I have upgraded Linux kernel to
2.6.32, hoping that maybe the problem is in software RAID driver, but no
changes, occasionally I still get errors. I still have to do memory test on
the server, but I doubt faulty memories are the problem, because otherwise
the server behaves well.