T-SQL (SS2K8) :: No Error When Sub Query Not Valid

I have a query that is coming back with all my rows from an "IN" where condition. The problem is that the query in the sub-query is invalid.

I can't recreate it exactly but in the sample here, I don't get any rows back (not sure why they are different) but why am I not getting an error?

IF OBJECT_ID('tempdb..#temp1') IS NOT NULL
DROP TABLE #temp1

IF OBJECT_ID('tempdb..#temp2') IS NOT NULL
DROP TABLE #temp2

CREATE TABLE #temp1

[Code] ...

The "SELECT LastName FROM #temp2" subquery is invalid and if you run it by itself you do get the error.

So why no error when this runs.

The other query is:

SELECT *
FROM Staging ves
WHERE ves.ssn IN ( SELECT ssn
FROM Employee )

There is no "ssn" in the Employee table (it is in a different table), I get 12,000 records back from Staging.

Why would that be the case?