The demotion of a DC was not successful or the DC crashed or was removed without ever demoting it, as it should, to a non-DC. For that reason its metadata still exists in AD and needs to be removed!

See the articles below for how to cleanup the metadata of a DC.

How to remove data in Active Directory after an unsuccessful domain controller demotion
--> http://support.microsoft.com/?id=216498

Clean up server metadata
--> http://www.microsoft.com/technet/prodtechnol/windowsserver2003/library/Operations/012793ee-5e8c-4a5c-9f66-4a486a7114fd.mspx

Delete extinct server metadata
--> http://www.microsoft.com/technet/prodtechnol/windowsserver2003/library/ServerHelp/1a7522c3-ac6e-4f83-af5b-9be87b47a95d.mspx

How can I manually delete a server object from the Active Directory database in case of a bad DCPROMO procedure?
--> http://www.petri.co.il/fix_unsuccessful_demotion.htm

How can I delete a failed Domain Controller object from Active Directory?
--> http://www.petri.co.il/delete_failed_dcs_from_ad.htm

 

Concerning an AD domain, you did not select "this is the last server in the domain" option during the demotion of the LAST DC in an AD domain or the last DC just got removed without demotion or you used forceremoval instead of a normal demotion. For that see:

Delete extinct domain metadata

--> http://technet2.microsoft.com/WindowsServer/en/Library/b399e7d2-8bc5-4fea-b84f-25f2bd09f5cc1033.mspx?mfr=true

How To Remove Orphaned Domains from Active Directory

--> http://support.microsoft.com/?id=230306