Major spoilers here - turn back now if you haven't read all the books:
I am in the middle of my second reading of book one and just realized that Jon Snow is not Eddard Stark's son.
"Treachery is a coin that the Tragaryen's knew well," Robert said. The anger was building in him again. "Lannister payed them back in kind. It was no less than they deserved. I shall not trouble my sleep over it!"
"You were not there," Ned said, bitterness in his voice. Troubled sleep was no stranger to him. He had lived his lies for fourteen years yet they still haunted him at night.
Note: Jon Snow is 14 years old in the books
And then when he went to visit Robert's whore in Littlefinger's brothel:
"I will," Ned had promised her. That was his curse. Robert would swear undying love and forget them before evenfall, but Ned Stark kept his vows. He thought of the promises he made Lyanna as she laid dying, and the price he paid to keep them."
Jon Snow is Lyanna's son - and presumably of Targaryen blood (Rheagar).
Also, I learned yesterday that Oberyn Martell poisoned Tywinn Lannister - he was going to die whether or not Tyrion shot him or not - which is why it only took a bolt to the groin to kill him and also why his corpse rotted away so quickly and pulled taut.
"Where will I find my lord father?”
“In the solar with Lord Tyrell and Prince Oberyn.”
“Widow’s blood, this one is called, for the color. A cruel potion. It shuts down a man’s bladder and bowels, until he drowns in his own poisons.”
“Your father,” said Prince Oberyn, “may not live forever.”
The King’s Hand was rotting visibly. His face had taken on a greenish tinge, and his eyes were deeply sunken, two black pits. Fissures had opened in his cheeks, and a foul white fluid was seeping through the joints of his splendid gold-and-crimson armor to pool beneath his body.