execute incorect query and get data

PostgreSQL : 8.2
Operating system: fedora RC4

--------

create table a (a1 integer);
create table b (b1 integer);

insert into a (a1) values (1);
insert into a (a1) values (2);
select a1 from b;

ERROR: column "a1" does not exist at character 8

but :
select a1 from a where a1 not in (select a1 from b);

a1
----
1
2
(2 rows)

when b is not empty :

insert into b (b1) values (1);

select a1 from b;
ERROR: column "a1" does not exist at character 8

but :
select a1 from a where a1 not in (select a1 from b);
a1
----
(0 rows)

select a1 from a where a1 in (select a1 from b);
a1
----
1
2
(2 rows)