This was very much a me mistake, so apologies from me.

The long story short, is that we were migrating the domain between accounts.

As the previous DNS records would be wiped during transfer, we made backups, and started the process.
This morning, the "approve transfer" button was lit.
A sensible person would have gone "perfect, lets get this scheduled".
Instead, I just pressed the button.

Unfortunately, when you transfer, and are keeping continuity, you should always make sure the receiving account is ready to re-apply the settings.
I didn't, and the receiving account owner was happily asleep.
100% my bad!

All sorted now (hopefully).
There might be some weirdness while DNS updates propagate through the various caches.

I think the lessons learned are pretty obvious:

• Consider how noticeable downtime will be, and schedule with this in mind.
• Check that your counterpart is online when transferring a domain.
• Always schedule a maintenance window, even if you get excited when the button activates.