Arya's murderous scene had a hidden reference: The Rat Cook.
Feeding Walder Frey's two sons to him in a pie was actually an adapted storyline from the book series. Though Arya (and Lord Walder) aren't the key characters involved in the books, the showrunners clearly had this plan in mind for awhile. A big hint about the Freys' cannibalistic fate was given back in season three.