Part of the problem is that so many devices are poorly behaved when it comes to DNS. At one point I worked for a company that had a large mobile app presence. W

Part of the problem is that so many devices are poorly behaved when it comes to DNS. At one point I worked for a company that had a large mobile app presence. We setup new authoritative name servers to conduct a test for a week or so. After the test was completed we removed the name servers records. A lot of clients went away very quickly... but way more stuck around way longer than they should have. At two months post test, those test servers were still getting some traffic.