I understand that KB2670838 is currently set as an optional update so it should not be downloaded or installed automatically... [emphasis mine]
And therein lies the rub, my friend.
Allow me to quote the OP in this thread...
I currently have laptop set to automatic download and installation of windows updates. Update KB2670838 causes system crash... [emphasis mine again]
IOW, it would appear that Automatic Updates is downloading/installing KB2670838 (at least as long as AU's set to the default Automatic setting).
AU's set to Notify Only here so I was able to hide it before AU offered it.
[PS: Running Fix It 50202 in Aggressive mode will "flush" the Download folder.]