Query returning tuples that does not satisfy the WHERE clause

Hi all,

I'm running PostgreSQL v 8.1.4 and found a query that returns tuples
that does not satisfy the WHERE clause, the query is:

select * into errores_20071 from (
select r.id, r.trayectoria_id, r.grupo_id, regacd.insc_registra_grupo(trayectoria_id, grupo_id, true, false, true) as error
from regacd.registro r join regacd.grupo g on (g.id = r.grupo_id)
where g.año_semestre = 20071 and g.tipo_id = 'a') x
where error is not null;

A self-contained database schema is here: