I've found a hint!

You know, all the Harry Potter books follow the same format: they cover one full year of school at Hogwarts, culminating in some climactic event (that centers around He Who Shall Not Be Named), and ending with Dumbledore explaining more details from Harry's family past in a heart-to-heart with him.

If you really want to find the hints, you can probably just read the last several chapters of each book.

Anyway, I found this one last night, at the end of the third book (Prisoner of Azkaban):

"Petigrew [Wormtail] owes his life to you. You have sent Voldemort a deputy who is in you debt...When one wizard saves another wizard's life, it creates a certain bond between them...and I'm much mistaken if Voldemort wants his servant in the debt of Harry Potter."

"I don't want a connection with Petigrew!" said Harry. "He betrayed my parents!"

"This is magic at its deepest, its most impenetrable, Harry. But trust me...the time may come when you will be very glad you saved Petigrew's life."

Ooooo!! FORESHADOWING!!