Cache lookup failed?

What might be the source of this error?

 Cache lookup failed for relation 188485009

We've been getting these at odd intervals, and they are not reproducible.

Our setup:

 PostgreSQL 7.3.3
 Red Hat 7.3

 kernel.shmall = 1352914698
 kernel.shmmax = 1352914698

 shared_buffers = 131072
 max_fsm_pages = 350000
 max_fsm_relations = 200
 wal_buffers = 32
 sort_mem = 65536
 vacuum_mem = 65536
 effective_cache_size = 196608

--
Jeff Boes                                      vox 269.226.9550 ext 24
Database Engineer                                     fax 269.349.9076
Nexcerpt, Inc.                                 http://www.nexcerpt.com
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