Installing a new Exchange 2013 deployment on an existing domain with questionable history. The existing domain had untold number of previous Exchange deployments with remnants scattered throughout Active Directory. After using ADSI edit to manually remove all instances of the old Exchange servers, running a schema prep runs in to an error about an existing Active Directory Connector. Microsoft suggests (TechNet) disabling the ADC service on the running computer, then uninstalling the service using the exchange server installation CD. In this situation of course the detected ADC was probably on a computer that was long ago trashed or removed.
Lots of websites describe the method for removing ADCs from AD using ADSIedit.