ejoell 66477 (9/21/2011)
George, in the original post dbajunior said "I uncheck db_owner, then click OK." Well in my version of SQL Server 2008, the login mapped to dbo is not listed in the users list of the databases except as dbo, you can not map a login that is already in the database as another user as dbo, and the in the user mapping area of the logins properties, trying to remove the check for db_owner generates an exception 'cannot use special principal dbo' so he would never be able to successfully click "OK".
it can happen if the user is defined in the database already and then that user creates the database by attach or restore, user will also become dbo.
---------------------------------------------------------------------