subquery abnormal behavior

I just noticed an abnormal behavior for the subquery:

create table myt1 (a numeric);
create table myt2 (b numeric);

select a from myt1 where a in (select a from myt2);

This should be giving an error that column 'a' does not exist in myt2 but it
runs with any error...

I had been trying it on 8.2!

Can someone please comment?

Thank you,
-------------
Shoaib Mir
EnterpriseDB (www.enterprisedb.com)