A bug in "not in" query

Please note the code block below:

There are two tables called outer_table and inner_table.

The table inner_table has two columns aband bc; but in sub query (first and second subquery) i have used it wrongly as selecta from inner_table.

I thought I would get a parse error. What I found is surprising.  For first query no result set and for second query, 4 records of value 2 were returned. I ran it in SQL server 2005 and SQL server 2008.

3rd and 4th query, though behaved

Is it not a bug? Or can some one explain how SQL behaves here?

CREATE TABLE Outer_table (a INT , b INT)
CREATE TABLE inner_table(ab INT ,bc INT)
go
INSERT outer_table SELECT 1,2
INSERT outer_table SELECT 1,2
INSERT outer_table SELECT 1,2
INSERT outer_table SELECT 1,2

INSERT inner_table SELECT 1,2
INSERT inner_table SELECT 1,2
INSERT inner_table SELECT 1,2
INSERT inner_table SELECT 1,2
GO

SELECT a FROM outer_table WHERE a NOT IN (SELECT a FROM inner_table)
SELECT b FROM outer_table WHERE b NOT IN (SELECT a FROM inner_table)
SELECT a FROM outer_table WHERE a NOT IN (SELECT c FROM inner_table)
SELECT b FROM outer_table WHERE b NOT IN (SELECT c FROM inner_table)