I was fighting the demi-lich with Galadhven, and I would turn undead, flee, then turn undead again. As I did so, I was informed at one point that I couldn't improve my skills at turn undead because the demi-lich was not affected by it. That was peculiar, since I was using it to a fair amount of affect, considering. I don't know that this would come up all that often, but hey, it might apply elsewhere too.

If you can't hurt it with your melee attacks, you can't learn.