BUG #4250: subquery in query

The following bug has been logged online:

Bug reference: 4250
Logged by: Wojciech Tylek
Email address: wat(at)wat(dot)net(dot)pl
PostgreSQL version: 8.1,8.2,8.3
Operating system: Linux
Description: subquery in query
Details:

There are two example tables:

create table a (
a_id serial primary key,
sth varchar
-- anything else
);

create table b (
b_id serial primary key,
a_id int references a(a_id),
sth int
--anything else
);

The question is:
Why the query like:

select * from b where a_id in (select a_id from a where b_id in (10,20));

does not return error?
The table "a" does not have b_id!
And for example a query (which is a subquery in above): select a_id from a
where b_id in (10,20);
returns ERROR: column "b_id" does not exist.