DNS has always been one of the weakest links in security. From transparent DNS proxing, to DNS hijacking in local network attacks, to complete and quite transparent, often very permanent and undetectable hijacks of consumer-level network equipment, such as routers, through trivial UPNP hacks.
Using this service you can tell weather your ISP is doing some bad things behind your back: dnsleaktest.com. Since more and more ISPs around the world starting proxifying DNS traffic through their own servers it’s becoming more and more of a problem. Some of them are actually modifying the result of any lookup of google.com regardless of if user is using their DNS or not.