Update failed? your kidding right?

Ok so let me start by saying I don't have the greatest computer, but it runs leagues near flawlessly. that out of the way I have NEVER gotten the message I just received, and I quote "An unknown error occurred, your update has failed." no error code, no weird thing or even a bugsplat which I routinely get when something is wrong with my client, that said what might have caused this cause I cant see a problem other than it reached 99% which pisses me off because it took so damn long to do, and that it basically farted out at 99% which never happens for me until now