ERROR: cache lookup failed for type 14237017

Yesterday, I had twelve thousand  "cache lookup failed for type N"
messages, like this:

2010-09-20 00:00:00 PDT ERROR:  cache lookup failed for type 14237017
2010-09-20 00:00:00 PDT CONTEXT:  SQL statement "INSERT INTO
mycluster.sl_log_2 (log_origin, log_xid, log_tableid, log_actionseq,
log_cmdtype, log_cmddata) VALUES (1, $1, $2,
nextval('mycluster.sl_action_seq'), $3, $4);"

The context is always Slony sl_log_2 table.  All twelve thousand
errors occurred within 40 minutes.  This did happen right after a
Slony cluster set drop and recreate.

What does "type 14237017" mean?  What cache are we talking about?

I am concerned because this is classified as ERROR rather than WARNING.

Thanks,
Aleksey