Many, many years ago, my department operated one of the university's secondary authoritative DNS servers, which was used by most everyone with a university subdomain and as a result was listed as one of their DNS NS records. This DNs server was also the authoritative DNS server for our own domains, because this was in the era where servers were expensive and it made perfect sense to do this. At the time, departments who wanted a subdomain pretty much needed to have a Unix system administrator and probably run their own primary DNS server and so on. Over time, the university's DNS infrastructure shifted drastically, with central IT offering more and more support, and more than half a decade ago our authoritative DNS server stopped being a university secondary, after a lot of notice to everyone.

Experienced system administrators can guess what happened next. Or rather, what didn't happen next. References to our DNS server lingered in various places for years, both in the university's root zones as DNS glue records and in people's own DNS zone files as theoretically authoritative records. As late as the middle of last year, when I started grinding away on this, I believe that roughly half of our authoritative DNS server's traffic was for old zones we didn't serve and was getting DNS 'Refused' responses. The situation is much better today, after several rounds of finding other people's zones that were still pointing to us, but it's still not quite over and it took a bunch of tedious work to get this far.

(Why I care about this is that it's hard to see if your authoritative DNS server is correctly answering everything it should if things like tcpdumps of DNS traffic are absolutely flooded with bad traffic that your DNS server is (correctly) rejecting.)

In theory, what we should have done when we stopped being a university secondary authoritative DNS server was to switch the authoritative DNS server for our own domains to another name and another IP address; this would have completely cut off everyone else when we turned the old server off and removed its name from our DNS. In practice the transition was not clearcut, because for a while we kept on being a secondary for some other university zones that have long-standing associations with the department . Also, I think we were optimistic about how responsive people would be (and how many of them we could reach).

(Also, there's a great deal of history tied up in the specific name and IP address of our current authoritative DNS server. It's been there for a very long time.)

PS: Even when no one is incorrectly pointing to us, there's clearly a background Internet radiation of external machines throwing random DNS queries at us. But that's another entry.