" . . . My - our - one stroke of good fortune was that the eavesdropper was detected only a short way into the prophecy and thrown from the building . . . He heard only the beginning, the part foretelling the birth of a boy in July to parents who had thrice defied Voldemort. Consequently, he could not warn his master that to attack you would be to risk transferring power to you, and marking you as his equal. "
-- Albus Dumbledore to Harry (OP37)
" . . . there was a commotion outside the door and it flew open, and there was that rather uncouth barman standing with Snape, who was waffling about having come the wrong way up the stairs, although I'm afraid that I myself rather thought he had been apprehended eavesdropping on my interview with Dumbledore - you see, he himself was seeking a job at the time, and no doubt hoped to pick up tips"
-- Sibyll Trelawney to Harry (HBP25)
When Dumbledore met at the Hog’s Head with Sybill Trelawney as a “courtesy” to interview her for the post of Divination Professor, she suddenly launched into a real prophecy about a child with the ability to vanquish the Dark Lord. (OP37)
Lurking outside the door and listening at the keyhole was a spy of Lord Voldemort, Severus Snape, who was caught before he heard the entire prophecy by Aberforth Dumbledore, who threw him out of the Hog’s Head. Snape hurried away to tell his master the partial prophecy he overheard, but Dumbledore explained to Harry that later it became the “greatest regret of his life and the reason that he returned” to the good side to try and save the Potters. (HBP25)