Yeah, I had a few of those myself, and unfortunately there's nothing to be done for it. Don't despair, though: you'll still have plenty of opportunities to get more dragon souls, and by the time you reach your higher levels you'll have souls to spare! I'm at level 81 now and I think at last count I'm sitting on over 50 dragon souls that I have no use for, having attained every word of every shout in the game. But you
can have a little fun with it though, if you feel like blasting a dragon corpse all over the place with Unrelenting Force for a few laughs!
