could not open relation with OID

I've seen the "could not open relation with OID" error a couple times
recently and Googling for that error gives responses ranging from *very
scary* to fuggetaboutit.

The error today occurred running this query which is part of a database
status reporting script:
select
pg_size_pretty(sum(pg_relation_size(oid))::bigint) as "Database
Table Size"
from
pg_class
where
relkind = 'r'
;

could not open relation with OID 81285213

Pre-answering some potential questions based on threads I read:

- Version is 8.3 x86_64 (about to be replaced with 9.0).

- Database appears to be functioning fine otherwise and query reran
without error.

- Size just under 100-million total rows and 12GB (counting tables only).

- Backups completing without error.

- Restore of backup to a twin machine completes without error.

- There are lots of scripts and processes that use temporary tables
(there are currently 35 pg_temp_XX schemas with the highest being
pg_temp_55)

So...

Worry or not worry?

Cheers,
Steve