There's a radiant Dawnguard quest called "A Jarl's Justice." What happens is, the vampire needing to be killed isn't there. The quest marker appears INSIDE a wall, very low to the ground. The target is not there. Talking the the Jarl doesn't trigger a vampire to come up either. So that needs to be fixed.
My other bug is that I am told Isran wants to talk to me. He doesn't it seems, even though it is an objective in my misc quests.