Albus Dumbledore would always count Tom Riddle and Sirius Black as the two greatest mistakes he had made with his students.
Tom Riddle set off alarm bells in Dumbledore's head the first time he met him. Tom could have been saved, had someone shown him the way. But as he charmed all the teachers other than Dumbledore, and Dumbledore did nothing but distrust him, Tom Riddle learnt that he could do anything as long as he didn't get caught. Dumbledore was sure there were many time's he didn't get caught.
Tom Riddle committed his first murder at sixteen.
He charmed his way through life, gathering his resources and learning all he could.
They were all fooled by him for years, and laughed with him as he secretly plotted his rise as Voldemort.
They were all shocked when he revealed his true self.
Dumbledore bowed his head in sorrow at the news.
Sirius Black set off alarm bells in Dumbledore's head the first time he met him. So he did what he did not with Tom, and acted as a kindly teacher that could be approached, and yet he fell to the charm of a smile and a wink like all the other teachers. Sirius Black learnt that he could get away with anything and that he could charm his way out of any trouble if caught. Dumbledore was sure there were many times he didn't get caught.
Sirius Black first tried to murder someone at sixteen. It would have succeeded had James Potter not figured out what was happening and rescued Mr Snape in time. And Dumbledore realised that he had fallen into the same trap, and viewed Sirius as not dangerous beyond petty pranks. And he couldn't expel the boy, because that would send him back to the Black family.
He charmed his way through the world, gathering information.
They seemed to have been fooled by him for years, laughed as he apparently plotted betrayal.
They were all shocked that fateful Halloween.
Dumbledore bowed his head in sorrow at the news.
He bowed his head one more time in that manner. It was a full moon night in young Harry's third school year, when he did so, but this time it was accompanied by words.
"I have failed you."
And Sirius Black met his gaze. His eyes burned with the pain of Azkaban and betrayal as he replied.
"Yes, you have."
For Sirius was innocent.