https://tools.ietf.org/html/rfc3658

1. DNSSEC/RFC/3658

について、ここに記述してください。

Such a situation might arise when a nameserver is authoritative for
both a zone and it's grandparent, but not the parent.  
This sounds like an obscure example, but it is very real.

The root zone is currently served on 13 machines, 
and "root-servers.net." is served on 4 of the 13, but "net." is severed on different nameservers.