This geek, I do not reveal his name, let's call him Firi Kann, posted on a tech site that he had installed build 9879 with a local account and then using this account only upgraded the computer and a virtual machine on it through every build through Windows Update Fast Ring.
He started to doubt his own words and fired up this PC, and when on desktop also the Hyper-V vm on it. This PC and its vm had been setup only for the purpose to see if upgrades work, and had never been used to anything else. Booted about once a week, let run an hour or so to get the updates and that's it. Upgrade test has worked, both machines always automatically upgraded to next build.
Mr. Firi Kann had completely forgotten that both machines, the real and the vm, had originally been installed using an MS account, then on both machines he had also created a local admin account without password so Windows always booted automatically to the desktop of this local account. As it is, he had to find a way to admit he was wrong, and not finding any suitable way to do that he decided to write a short story, blaiming someone else