Orphaned IPs as well. If you have an IPv4 from your cloud provider and you want to retire it, you should thoroughly scrub your DNS and all other configs before doing so. Otherwise it's trivial for someone else to spin up a machine on that IP address and abuse your domain.

Yeah those orphaned domains are a goldmine for security researchers, there was a similar talk at blackhat where they showed how expired domains from major companies still recieved auth tokens and sensitive data for months after expiry.

Please post a link if you're able, that sounds like a very interesting watch.