Recently I was installing a windows update and the windows update dialog indicated a successful install.

After having trouble with another update afterwards, I checked the event log and found this error:

update guid{B78C39B2-C915-4F8C-9780-23F71C14A560} ErrorCode:0x80070bc9

This error can apparently have many causes and solutions. 

The solution that worked for me, however, was to simply delete the contents of the %systemroot%\windows\winsxs directory with the idea that several updates had not downloaded successfully. Download a cmdlet fix below:

After doing so, I ran windows update again and all the updates installed successfully without event log errors.