Sorry, but I lost my original query, and I've forgot exactly the problem. Instead of give up, I want to report it anyway, based upon what I still recall.
The problem was like this:
I had a query of the type

IF (EXISTS (SELECT * FROM ATABLE
INNER JOIN BTABLE ON (ATABLE.MOVCON_ID = BTABLE.MOVCON_ID)
WHERE (CONDOMINIO_ID=:w_CondominioId)

That worked, but did not did what I wanted. To test it I've tried then select alone, i.e.
SELECT * FROM ATABLE
INNER JOIN BTABLE ON (ATABLE.MOVCON_ID = BTABLE.MOVCON_ID)
WHERE (CONDOMINIO_ID=:w_CondominioId)

But then FB 1.0 told me that CONDOMINIO_ID was ambiguous (it belongs to ATABLE and BTABLE).
So seems that the ambiguity check is not done inside EXISTS?
Can you confirm this? Is it a bug?
Thanks
Marco Menardi