A story from the epics
In the Mahabharata war, the Pandavas could not defeat their teacher Drona. Bhima killed an elephant named Ashwatthama and shouted, "Ashwatthama is dead!" Drona's son was also called Ashwatthama. Drona did not believe Bhima. He asked Yudhishthira, who was known never to lie. Yudhishthira said, "Ashwatthama is dead," and then said very softly, "the man or the elephant." Drona did not hear the last words. He believed his son was dead and put down his weapons.
Drona did one smart thing: he checked with a trusted source. But he did not hear the full message, and he did not ask a second question. Even a trusted source can give you only part of the truth.
Source: Mahabharata, Drona Parva. Details such as the conch drowning out the last words come from popular retellings.
