I was using Windows 10 Home, version 1607 (OS build 14393.693).
The computer was a bit slow earlier today. Task Manager indicated that "Service Host: Local System" was making very heavy use of the hard drive bandwidth; it looked like the machine was probably disk-bound, not CPU-bound. Sysinternals Process Monitor indicated that this was probably because of some Windows Update activity.
I opened the Settings app and realized that this was correct. Windows Update was downloading: Cumulative Update for Windows 10 Version 1607 for x64-based Systems (KB4013429).
It said "Downloading updates 95%".
And it continued to say "Downloading updates 95%", for hours.
Was it stuck?