Error dropping tables: "ERROR: could not find tuple for trigger "

Hi all,

   I have problems droppping some tables in a DB running on postgreSQL
7.4.2. Below you have an example of what is happening:

coltr_demo=# drop table agenda_users_groups;
ERROR:  could not find tuple for trigger 112070831

   I haven't found anything about how to solve this, anybody knows?

Thanks
--
Arnau