Then the unthinkable happened: rushing to see Yudhishthira, Arjuna walked in on Yudhishthira with Draupadi. Because he violated their brotherly agreement, Arjuna went into self-imposed exile.
It wasn't really Arjuna's fault. To ensure Draupadi's privacy when she was with each husband, the current husband would leave his shoes outside the door as a sign that he was inside. A dog, however, had stolen Yudhishthira's shoes, which is why Arjuna walked in unsuspecting.
In her rage, Draupadi cursed all dogs. "You will copulate in public for all to see!" she decreed. "That will be the shame of dogs henceforth and forever."
Inspired by: Jaya: An Illustrated Retelling of the Mahabharataby Devdutt Pattanaik.
Notes: This story is on p. 116.