PGError: ERROR: could not open relation with OID?

Running Postgresql 9.0.5 on Ubuntu 10.10, we just saw:

PGError: ERROR:  could not open relation with OID 39008

SELECT questions.*, r.relevance as score, r.explanation as explanation
FROM "questions"
INNER JOIN "users" ON "users"."id" = "questions"."user_id"
JOIN relevance(13218) AS r
ON questions.id = r.question_id
WHERE (questions.user_id != 467) AND (questions.user_id != 13218)
ORDER BY r.relevance DESC
LIMIT 10 OFFSET 0

This same query run manually from psql doesn't yield any error, so it must
have been a transitive thing.  We use no explicit locking statements, we
weren't creating or dropping tables and haven't in the past week or so, and
we have no replication, sharding or partitioning set up - it's pretty much a
stock single-machine install, save for the typical postgresql.conf RAM
tweaking.  Our volume is fairly low, the tables are fairly small, we have
complete logs.

How can I troubleshoot to see what might have happened?  Or is this a known
bug in 9.0.5?

Jay Levitt