Gregory, I think Liberius actually remained Pope and Felix was the Anti-Pope, according to most theologians who studied the issue and even perhaps official Church lists. That being said, I think St.Robert Bellarmine's comments on it are illumininating, for he says the opposite also applies.
He says what Raoul said, that men are not bound to read hearts, and even if they are wrong, in being sedevacantists, as it would seem those who attempted to declare Pope Liberius deposed were, God would not hold that alone against them.
Of course, even as someone who does believe there is a Pope, I grant the common teaching of theologians on this point, that a Pope cannot be a manifest heretic. However, I still think a future ecclesiastical court will have to decide the question, for all faithful Catholic Christians to have the necessary certainty we need in such a matter.
Obviously, even if the good God, in His mercy, will choose to forgive us, we must still act in accordance with known truth. But I think, frankly, it is hard, to be sure about this, either way or the other. I've tried. Now, I prefer to leave it to God, and devote myself to prayer and the spiritual life. If at all I'm wrong, and it is God's will for me to know the truth in this matter, I trust that in any case prayer will help me in coming to that knowledge more than theology alone.