Towards the end of Half-Blood Prince, Sybil Trelawney mentions having warned Dumbledore that the cards kept showing her the "Lightning-Struck Tower." The chapter in which Dumbledore is killed is actually called the Lightning-Struck Tower, and Dumbledore is killed on top of the Astronomy Tower by the same curse that gave Harry his lightning-shaped scar.
Was she making a real prediction here?
To paraphrase an earlier answer I gave, yes, I believe this is a legitimate example of divination.
Earlier in the book, Harry sees Trelawney when she thinks she's alone using her cards, and they keep showing her that danger is coming.
Harry proceeded through deserted corridors, though he had to step hastily behind a statue when Professor Trelawney appeared around a corner, muttering to herself as she shuffled a pack of dirty-looking playing cards, reading them as she walked.
"Two of spades: conflict," she murmured, as she passed the place where Harry crouched, hidden. "Seven of spades: an ill omen. Ten of spades: violence. Knave of spades: a dark young man, possibly troubled, one who dislikes the questioner —"
She stopped dead, right on the other side of Harry's statue. "Well, that can't be right," she said, annoyed.
Obviously that prediction is right on the money. Draco Malfoy, a troubled young men, will soon bring violence upon Hogwarts.
So when Trelawney is using the same cards and continues to see the lightning-struck tower "again and again, no matter how I lay them out," I think it's far to assume Trelawney has finally struck upon a legitimate form of divination. The chapter title is just icing on the cake.
