#8: The textbook fluke
A number of stars had to align for Harry to wind up in possession of a vitally important copy of Advanced Potion-Making in The Half-Blood Prince. First, he had to incorrectly assume that he wasn’t eligible for the N.E.W.T’s potions class, leading him to not purchase his own copy. Slughorn then had to select that specific old book from the pile for Harry. Ultimately, Harry having that particular textbook led to him saving Ron’s life with a bezoar after he's been poisoned, and helped Harry to win a bottle of Felix Felicis during his first class. This in turn later proved essential in helping him acquire a memory from Slughorn about Tom Riddle and Horcruxes.