Bit of background for folks that don't know me well:
1) Over the years I've done any number of different jobs in and around the IT sector, and many of them have included being the interface/interlocutor between groups with different levels of technical expertise and knowledge. As such, I've gotten to be a fairly dab hand at explaining client needs to engineers and technical requirements to end users (clients, if you're big on that sort of framework), which has led - for my sins - to writing a lot of documentation. I've never had any official training, but "Dude, you've got 3 days to learn how this works and write an FAQ before we launch, sorry" is a powerful motivator, so it's become a fairly common element of any job I end up in.
2) Some time back, I had the dubious honor of hosting a novel bacterium in my gut that caused sepsis, coma, sorta-kinda-maybe-but-not-quite-dying and consequently, a certain amount of cognitive/memory impairment (mostly narrative memory gaps past & ongoing and aphasia, which continues to be a minor annoyance to this day.
::End of necessary context::
A few weeks back, for whatever reason, somebody asked me a question about DNS subdomain delegation to secondary name-servers and I realized I'd hit a total blank spot in my subject knowledge. To be fair, the last time I'd had to deal with it was at least a decade ago, so I'd expect to not recall the particulars, but I was drawing a complete blank even on the conceptual aspect of it, which was weird, since I was fairly sure I remembered knowing it previously.
Anyways, I hopped over to my account and zone file settings at the DNS hosting company I used to work at (and where the domain & authoritative records still reside), and poked around the interface, but just couldn't figure it out. So I gave a shout to the support line and ended up chatting with my old manager. Ran the issue by him, and asked if he could point me to what I was doing wrong. He snorted in a friendly/teasing tone, and said "Why don't I send you the FAQ so you've got it as a reference if you need it in the future?" I said sure, and we ended the call.
Couple of minutes later, I get an email from him with an attachment, and I start reading it. As I do, I realize a few things :
I had forgotten that the ability to delegate subdomains required the more advanced feature-set that came with a slightly higher grade account type
The FAQ was over a decade old.
My name was at the bottom of the document as having written it.
I'm sure there must be a German word for the feeling that covers "having a professional acquaintance have to remind you of work you've already done to answer a question you wrote the documentation to answer in the first place", but the closest I can come is "Wow, past me was a LOT more on the ball than I am, thanks dude!"
Anyways, as always, it's probably the DNS that's the problem