Hassan's death, for obvious reasons. How Amir couldn't see him one more time and apologize. It's even worse because before dying he wrote to him an affectionate letter and has always spoke highly of Amir to gis son.
Hassan's mother return and how he forgives her for having abandoned him, the relationship they mamage to repair and how her little grandson becomes the centre of her life. It's a far cry from the beautiful and tumultuous young woman she's said to have been.