I'm playing as a vampire with 100% spell absorption and since I installed Dawnguard some spells have started to pass through. I was experimenting with a legendary dragon when I observed that his Drain Vitality shout was never absorbed and further testing indicated the damage wasn't affected by the regular resists either. Also his fire breath was also passing through from time to time. I did also some tests with traps (in order to have spells cast repeatedly at me) and indeed, from time to time, some spells were not absorbed. I don't know if it's a bug, if a cap for absorption was introduced or if the spell absorption works like a ward and it can absorb only a limited amount of damage.