thanks, ynsaen. It's interesting that you say its the back side of the polygons poking through the bottom of the lip. I ticked the "remove backfacing polygons" in the render settings prior to doing the render. Also, making the teeth transparent didn't make any difference, and there was no "gums" material group. Although there was an "inner mouth" group, and the black spot went away when I made this transparent. So, what happens when I want to pose Dork with his mouth open ?