Purpose of Tombstones and Ancestor Ids

Does anyone know what tombstones and ancestor ids are used for? We are receiving the following error in our directory server v 5.1 when adding ou=tgroup6

NSMMReplicationPlugin - _delete_tombstone: unable to delete tombstone nsuniqueid=4a4d6c81-1dd211b2-8068bd2b-ede6b877, cn=tgroup6,ou=groups,ou=EIP,ou=apps,dc=test,dc=com, uniqueid 4a4d6c81-1dd211b2-8068bd2b-ede6b877: Operations error.

ancestorid BAD 13120, err=-666 Unknown error: -666

We have added and deleted ou=tgroup6 several times as part of our test group but just recently got this error. Did one of the deletes not remove this ancestor id?

Thanks!

[647 byte] By [709567] at [2007-11-25 8:06:07]