Chapter 27 in "Harry Potter and the Half-Blood-Prince" is called "The Lightning Struck Tower". In this chapter a certain character dies on top of a tower. The tower has the Dark Mark conjured above it. There is no mention in the chapter of the word "lightning", and the chapter has nothing to do with it. Why is this chapter called this way?
It's a callback to chapter 25, where Trelawney pulls out a card - the lightning-struck tower - and proclaims that calamity is coming:
“The headmaster has intimated that he would prefer fewer visits from me,” she said coldly. “I am not one to press my company upon those who do not value it. If Dumbledore chooses to ignore the warnings the cards show —” Her bony hand closed suddenly around Harry’s wrist. “Again and again, no matter how I lay them out —” And she pulled a card dramatically from underneath her shawls. “— the lightning-struck tower,” she whispered. “Calamity. Disaster. Coming nearer all the time...”
Harry Potter and the Half-Blood Prince, chapter 25: "The Seer Overheard"
Trelawney has apparently been predicting disaster, after pulling out her cards and this one showing up again and again. And, as it turns out, she was correct.