When trying to install KB3081424, it fails at a certain percentage and triggers a computer reboot. Once the user logs back in the operating system, the update attempts to install once again with the same result, so the exact same behavior is experienced over and over again.
The installation fails with the following error: "We couldn't complete the updates, undoing the changes." Since essentially all Windows 10 users at this point are not behind an official Update Server, the patch gets offered again.
Such are the vagaries of forced updating. The error repeats, again and again. About all you can do is get your work done before Win10 Update tries to install KB 3081424 and crashes again.
There are various similar reports on the Microsoft Answers forum. One very convincing report comes from Microsoft MVP and PowerShell expert Richard Siddaway, who blogged on the PowerShell for Admins site:
Microsoft rolled out a Cumulative Update for Windows 10 -- KB3081424 today. It downloaded and tried to install -- got about 30% of the way through, rebooted and then went through a number of reboot cycles to uninstall the changes.
The solution that seems to work involves a registry edit to delete stunted user profiles that shouldn't be there. Here's how to troubleshoot your system:
Step 1. Click inside the Cortana search box and type regedit. When "Regedit / Run command" appears at the top of the list, press Enter. (Or you can click or tap.)
Step 2. Inside regedit, on the left, navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
Step 3. Important! Back up the registry key. With ProfileList highlighted, click File, Export. Choose a location (like your desktop) and type in a name (like ProfileListBeforeIBrokeIt) and press Save.
Step 4. On the left, you'll see a list of profile IDs, most likely including S-1-5-18, S-1-5-19, S-1-5-20, and several more with much longer names. Chances are good one of them represents a bad profile that should've been deleted.
Step 5. Click on each of the long-name profiles in turn, and look on the right for ProfileImagePath. If any of the entries in ProfileList point to a ProfileImagePath that doesn't make sense, right-click on the profile ID and choose Delete.
Step 6. "X" out of regedit. Reboot your machine. If something goes horribly wrong, double-click on the file you created in Step 3 to restore the Registry. (You can try the whole process over again, but choose a different ProfileList.) If all is well, click Start, Settings, Update & Security. KB 3081424 should be there, and will most likely start installing again while you're watching.
I've seen several culprits blamed for leaving behind bad profiles, but haven't been able to narrow it down yet. My guess is several programs or utilities chop off profiles and leave them dangling, and the afflicted ran those programs under Win7 or Win8, then upgraded in place to Windows 10.
The problem, of course, isn't only the never-ending update. The real problem is that we don't have tools to block the patch on affected machines.
If Microsoft ever tells us what happened -- or if it reissues KB 3081424 -- I'll let you know.