Jaime pushes Bran out the window (Episode 1, "Winter Is Coming")
Whether you first encountered it in the books or the show, your reaction to this moment probably went something like this: "Hmm, this is a pretty interesting fantasy world, I guess I'll stick with it to see whaHOLY SHIT, HE DID WHAT?!?!" The attempted murder of a child to cover up royal incest is the first of the series' countless big shocks, and the place where it establishes its bonafides as a very adult take on the epic-fantasy genre. It's also magnificently cruel in its casualness: "The things I do for love," sighs sisterfucking/kingslaying Jaime Lannister to his slightly too-beloved sister Queen Cersei as he tosses ten-year-old Bran Stark to his likely death for having caught them in the act. Who says romance is dead?