Windows 10 Pro SP 1511 upgrade warning

Heads up! If you have the built-in Administrator account enabled, make sure you add a standard account on the PC to the Administrators group before upgrading to Windows Service Pack 1511 (Build 10586.318). The SP doesn't respect the existing Administrator account status, and resets it to the default of "disabled", potentially locking you out of Administrative access to the PC! However, if you have another account added to the Administrators group beforehand, you can correct this after the fact. But if not, you are up the proverbial creek! This only applies to Windows 10 Professional Edition, not the Home Edition, which has no built-in Administrator account anyway.

Note: I've heard it said that it may be possible to boot into Safe Mode and logon as Administrator that way, regardless of whether the account's disabled or not, but I haven't personally tried it out to confirm.