When importing a database in your SQL instance you would find yourself with
Cannot use the special principal 'sa'. Microsoft SQL Server, Error: 15405 popping out when setting the sa user as the DBO of the database. To fix this,
Open SQL Management Studio and Click New Query. Type:
exec sp_changedbowner 'sa', 'true'
Close the new query and after viewing the security of the sa, you will find that that sa is the DBO of the database.