The Bodhisatta was once a conch-blower. He went with his father to a public festival, where they earned a great deal of money. On their way through a forest infested with robbers, the son warned his father not to blow on his conch shell, but his father persisted, and they were plundered by the robbers.
The story was told to a self willed monk who is identified with the father. J.i.284.