Unfortunately, despite the anycasting, advanced caching, and extensive security features, Google Public DNS is not the ideal DNS service. For one thing, it's not called “experimental” for nothing. From my home, I can’t reach 8.8.8.8. Packets end up ping-ponging between the addresses 9.9.9.18 and 9.9.9.17. Apparently some routing engineer at Google is a bit dyslexic.

via Google Public DNS service not ideal for everyone.