The time had finally come: Karna and Arjuna were going to fight.
Karna had long waited for this moment, and so had the naga Ashwasena. When Khandava forest burned, Arjuna killed all the nagas fleeing the fire. Ashwasena alone survived, and he now turned himself into a serpent-arrow inside Karna's quiver, ready to kill Arjuna.
But when Karna fired at Arjuna's head, Krishna lowered Arjuna's chariot. The serpent-arrow knocked off Arjuna's crown; it did not hit him.
When Ashwasena begged Karna to try again, Karna refused. "I cannot use the same arrow twice. I must kill Arjuna some other way!"
Inspired by: Jaya: An Illustrated Retelling of the Mahabharata by Devdutt Pattanaik.
Notes: This story is on p. 273. As Pattanaik explains, "Arjuna's arrows had pierced Ashwasena's mother, but he had survived, for he was then still in her womb."