Hm.  I seem to be having an issue with 3D motion blur during my animations.  When it's on, there's artifacts that appear on my models that aren't there while the motion blur is off.  See below:

As you can see, there seems to be a black smudge on the left side of the nose. as well as a angled line (though it's almost not visible in this picture) on the right side of the lower lips.

I've tried many different rendering options and, short of abandoning 3D motion blur, have not been successful at getting rid of these artifacts.  During an animation, they seem to jump and dance all around the models.

What is causing them?

Thanks a bunch!