Spoilers for Deathly Hallows:
This question about why Harry has a scar when Avada Kedavra leaves no trace got me to wondering. Why would Harry have a scar at all? If Lily's protection prevented Voldemort from touching Harry why could the piece of Voldemort's soul touch Harry at all? Why wasn't it destroyed on contact like Quirrell was later?
His mother died in the attempt to save him -- and unwittingly provided him with a protection I admit I had not foreseen.... I could not touch the boy. - Voldemort
-Goblet of Fire
Quirrell raised his hand to perform a deadly curse, but Harry, by instinct, reached up and grabbed Quirrell’s face –
Quirrell rolled off him, his face blistering too, and then Harry knew: Quirrell couldn’t touch his bare skin, not without suffering terrible pain – his only chance was to keep hold of Quirrell, keep him in enough pain to stop him doing a curse.