When Arjuna saw Bhima attacked by King Bhagadatta's elephant army, he ordered Krishna to drive his chariot into the fray.
As soon as Bhagadatta saw them coming, he unleashed a terrible weapon: a Vishnu-astra. Before Arjuna could shoot an arrow in defense, Krishna leaped up and the Vishnu-astra struck him in the chest.
It turned into a garland of flowers.
Krishna then explained. "Bhagadatta received that astra from Bhudevi, the earth-goddess, and she received it from Vishnu when, in the form of the boar Varaha, I pulled her from the sea. I created that astra; only I could stop it."
Inspired by: Jaya: An Illustrated Retelling of the Mahabharata by Devdutt Pattanaik.
Notes: This story is on p. 253. You can read about Vishnu's boar incarnation at Wikipedia: Varaha.