It looks like what happened is the polygons on the inside of the mouth have been “exploded”. (looks like a neckline to me, correct me if I am wrong)
It happens to me alot when re-topoligizing a model, and it can propogate through all subdivsion levels if you don’t catch it…common on fingers, toes, eye sockeets and mouths. Play with projection strength and projection distance when projecting details onto a re-topo’d mesh. (If that’s what you did when the problems popped up)
—What you have to do is move the points back to their original locations.
There may be better ways of doing this, but here’s a few things to try:
*Turn off double-sided display, flip normals until you see the problem area, and try grabbing and dragging the verts back into place, or use the smooth brush. I also read recently that the move tools can also be used, that I haven’t tried, but may work better.