There are services that remove certain (unwanted) domain names from the DNS system. Reasons for that are for example filtering access to adult websites, and similar things. Those are used in schools and homes.
This violates the DNSSEC standard since those missing domains cannot be cryptographically proven. Most of these services filter all DNSSEC signatures which make those name servers unusable.
Known Services
OpenDNS
Strips all DNS signatures and is therefore entirely unusable.
Alternative Name Servers
This wiki has a list of usable name servers: List of Public DNS Servers