It's very rare that I have an update fail, and usually a failed update works on the second try.
There was one earlier this year that failed to install repeatedly. It took some searching on many different fora to find the answer. It turns out that this particular update would fail to install if Windows was not booted with its own bootloader. I dual boot my main PC with Linux (on separate drives), and I was using grub to choose which OS to boot from. It's much less annoying than mashing F12 to enter the MB's boot drive menu. As soon as I booted directly from the Windows drive, the update installed perfectly.
Silly MS. Why should it matter which bootloader I'm using? It's a good thing that the two OS installs are on separate disks. If they were on the same disk, I would have had to restore the Windows bootloader to the MBR, install the update, and then reinstall grub so that I could boot Linux again. All for one Windows update.