ERROR: could not open relation with OID

This morning we got the following error from a daily script that
produces a simple largest-table report:
ERROR: could not open relation with OID 597597503

I reran the script and it completed without error.

Our server is running 9.1.15 from PgDg Ubuntu repos and the query run by
the script is:

SELECT
     relname AS TABLE,
     lpad ( pg_size_pretty ( pg_relation_size ( oid ) ), 9 ) AS SIZE,
     ( 100 * pg_relation_size ( oid ) /
             ( SELECT
                 sum ( pg_relation_size ( oid ) )
             FROM
                 pg_class
             WHERE
                 relkind = 'r' ) ) ::numeric ( 4, 1 ) AS percent
FROM
     pg_class
WHERE
     relkind = 'r'
ORDER BY
     relpages DESC
LIMIT 60;

My research hasn't led to a definitive conclusion on how to trace this
issue and how much I should be concerned about this error.

If it is of importance, it appears that a temporary table and temporary
index were being created within the same second that the query was run.

Any advice?

Cheers,
Steve