When an NS record points to an incorrect host. This can be caused when:

• A zone is delegated to a server that has not been properly configured to be authoritative for the zone.
• A server that is authoritative for the zone has an NS record that points to another that is not authoritative for the zone.

This will cause resolvers to direct queries to servers that will not respond authoritatively, if at all. This causes unnecessary network traffic and extra work for servers. 1/4th of all zones have lame delegations (source Domain Health Survey).