On act 1, when you find the stranger (Tyrael) in the crater, right after the Skeleton King, you bring him back to town and speak to Cain. Cain seems to not recognize him, but how is it possible, since in Diablo 2 act 4 theyre standing side by side and if I'm correct Cain knows Tyrael personally... Is this a game mistake?
Human Tyrael really looks nothing like Angel Tyrael. They do have the same voice, but there's no way you could tell that it's the same guy just by looking at them. The old Tyrael had a completely dark face, you could not see it.
Cain does have memory problems. See how many bags he has lost, with his working notes, all around the world.