I'm serious.
I was sneaking around a bandit camp trying to take out enemies with a bow and arrows.. One of the bandits was low on health. I went up behind him, and pressed the attack button. The first-person neck-slice-from-behind killmove played, only without a weapon performing the slice.
"Sneak attack 3.0x damage!"
Sadly, I can't provide a video.
EDIT: I just did it a second time, this time by pressing the alternate attack button.
EDIT2: I don't know how this is happening, but now it's happening all the time.