POSSIBLY SPOILERS:

After the fire giantess leaves the cave and you drop down through the serpentine ledges, there is a little lava pool where you learn the BURN spell. Once you regain control, you are meant to go forward through the brairs, but if your curiousity takes you backward, leaping back up through the serpentine ledges to the open cave, and you should fall into the lava, you'll be resurrected back at the little lava pool. You'll be prompted to learn BURN again, but since you've already learned it, it does not appear as a useable spell onscreen. Since the game is waiting for that specific response, you cannot open your spellbook to use the BURN spell. You are stuck in the bowels of the lava cave... forever.