Ok, here is another one:
1. I add a login to my database server:
EXEC sp_addlogin @Username, @Password, @Database;
This works fine!
2. I add a user, to a database by using the stored prcedure sp_adduser:
EXEC sp_adduser @Username;
This also works fine!
3. I want to remove the user from the database. Therefor i use the stored prcedure sp_dropuser:
EXEC sp_dropuser @Username;
This removes the user BUT what you'll see while digging deeper is that sp_adduser has created an SCHEMA and sp_dropuser don't cares a s%#t about that - it's still there after calling sp_dropuser :-(