Verifying the last fix meant actually using it — create a test subdomain, give it real records, confirm they land correctly, then clean up afterward the way every test fixture on this project gets cleaned up. The cleanup step is usually the boring part. This time it wasn’t.
Removal remembered the wrong inventory
Deleting a subdomain has always meant deleting one thing: the address record that pointed its name at the server. That was a complete inventory for as long as an address record was the only thing a subdomain could ever have owned. It stopped being complete the moment a subdomain could also own SPF, DKIM, and DMARC records of its own — new belongings, added by a brand-new feature, that the old moving-out checklist had never been told to look for.
Nothing about the removal code was wrong for what it knew about. It just didn’t know about the new things yet, because those things didn’t exist when it was written. That’s the ordinary shape of this kind of bug: not a mistake, a checklist that quietly went out of date the moment a sibling feature expanded what could be there.
A second bug hiding inside the fix for the first
Writing the fix meant matching four different record shapes instead of one, and the natural way to write that is a chain of “or this, or this, or this.” That natural way is also a trap here: added carelessly, an “or” doesn’t nest inside the existing “and this is the right zone” boundary — it sits next to it instead. The result would have been a delete that reached past its own subdomain and into whichever other zone happened to have a record with the same name. An easy mistake to make, caught before it shipped, by stopping to ask what the generated query would actually say.
The fix for that isn’t cleverness — it’s grouping. Put every “or” inside its own parentheses first, so the zone boundary still wraps the whole group instead of just the first clause. The kind of thing that’s invisible in the code and completely different in what it actually does.
What actually finished
Removing a subdomain now takes everything that subdomain came to own, not just the one thing it happened to own when the removal code was first written — and takes only that subdomain’s own things, nothing borrowed from a neighbor by an unlucky trick of query syntax.