They go to hell. Duh. ;-)
Seriously, it is an illicit practice, which means it is against the rules of the Church. And since we believe Jesus told us to listen to the Church to willfully and knowingly disobey would be a sin.
On this particular point, the idea is that the priest and the people serve two different functions. Some parts of the Mass are for the priest to say, some for the people and some together. This blurs the distinction between priest and laymen. It is the priest here offering the sacrifice on behalf of the people assembled.
Fair enough. :-)
I believe it is ok, nee encouraged for individuals to mouth the words of the mass silently along with the Priest. At least I have read this in the rubrics of the latin mass booklet.
But you bring me to something that is fast becoming a pet peeve of mine. The crossing of the forehead, lips, and heart before the Priest or Deacon reads the Gospel. I used to think it was a fine thing to do, but I read a thread a while back that said it was nothing but aping the reader like a monkey. That made sense to me because why would I cross MY lips when somebody else is about to read. So, I cross myself while everybody else does the whole pantomime. I just wonder if it is required to do that.
I figure this comes from Psalm 19, "May the words of my mouth and the meditations of my heart be acceptable in thy sight, O Lord, my strength and my redeemer."