Windows Update is pretty brutal in general.

While Windows 7 was still current, there was an update that refused to install, and reported a cryptic error code in the UI.

After digging into dense log files and online support I came to discover it was refusing to install because of an Incompatible version of the super-old Novell Netware client, that lived in C:\Old_Computer\hard-drive\installers\novell_installer-${someversion}.exe

Windows was scanning the entire hard drive @ install time to look for any software on it's list of incompatible software.