Mysterious 7.0.3 error

Greetings!  We have a script updating our database with thousands of
entries on a daily basis.  To speed up processing, we drop a
consistency check trigger before the update and recreate it
afterwards.  Occasionally, we get the following, even though the
database has no other live connections, and the trigger drop is the
first statement:

drop trigger rprices_insupdel on rprices;
DROP
ERROR:  RelationClearRelation: relation 160298 modified while in use


Any pointers most appreciated!

-- 
Camm Maguire                             camm@enhanced.com
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"The earth is but one country, and mankind its citizens."  --  Baha'u'llah