Harry Potter and the Deathly Hallows
How we cite our quotes:
"Can you forgive me?" he said. "Can you forgive me for not trusting you? For not telling you? Harry, I only feared that you would fail as I had failed. I only dreaded that you would make my mistakes. I crave your pardon, Harry. I have known, for some time now, that you are the better man." (35.45)
Dumbledore feels terrible for not telling Harry about the Hallows and for not being honest with him – and perhaps it was egotistical of him to assume that Harry would make all the same mistakes he did. However, now that Dumbledore has realized that he was wrong, he's learned from his mistake.
Hands, softer than he had been expecting, touched Harry's face, pulled back an eyelid, crept beneath his shirt, down to his chest, and felt his heart. He could feel the woman's fast breathing, her long hair tickled his face. He knew that she could feel the steady pounding of life against his ribs.
"Is Draco alive? Is he in the castle?"
The whisper was barely audible; her lips were an inch from his ear, her head bent so low that her long hair shielded his face from the onlookers.
"Yes," he breathed back.
He felt the hand on his chest contract; her nails pierced him. Then it was withdrawn. She had sat up.
"He is dead!" Narcissa Malfoy called to the watchers. (36.13-17)
This final betrayal of Voldemort is what undoes everything – Narcissa Malfoy chooses her love for her son over her loyalty to her former Lord. It's an echo of Snape's betrayal of Voldemort, and Regulus Black's. Where the Dark Lord goes wrong every time is in failing to recognize the real importance of love.