I believe what you saw was an optical illusion. If you refer to this link, you'll see that they used a mannequin for that shot. http://www.theforce.net/humor/captioning/cap38.shtml
If you look closely you will see that it is not his lips that move, it is just the flames around him that move, and make it look like he is moving. Watch it, watch it now and you will see, ha ha
It may be possible, although I'm not sure, the flames could trigger some nerves in the lip. However, it could be the dummies lip melted, and warped from the flames. I didn't notice this one though. Good spot!!
This is a true genuine slip-up. It almost looks as if he's breathing. Maybe when Lucas previewed the movie he took a restroom break or something...because this should have easily been caught.