If you try to extend the Schema for SCCM 2012 you may receive the following error messages:

Modifying Active Directory Schema – with SMS extensions.
DS Root:CN=Schema,CN=Configuration,DC=contoso,DC=com
Failed to create attribute cn=MS-SMS-Site-Code. Error code = 8224.
Failed to create attribute cn=mS-SMS-Assignment-Site-Code. Error code = 8224.
Failed to create attribute cn=MS-SMS-Site-Boundaries. Error code = 8224.
Failed […]