In book one of Percy Jackson & the Olympians, The Lightning Thief, Percy is selected to visit the Oracle and to lead the quest to discover Zeus' stolen lightning bolt. When he meets the mummified oracle, he receives this prophecy:
"You shall go west, and face the god who has turned,
You shall find what was stolen, and see it safely returned,
You shall be betrayed by one who calls you a friend,
And you shall fail to save what matters most, in the end."
This prophecy unravels throughout the story.
2. Percy later finds Zeus's lightning bolt in a backpack given to him by Ares. He returns it to Zeus, as well as Hades' Helm of Darkness, which was also in the possession of Ares.
4. In the end, Percy failed to save his mother, which mattered most to him, even though she was given back when Percy returned Hades's Helm of Darkness.