Hermione Refers Back To The Philosopher’s Stone
During the Battle of Hogwarts, the trio realize that the branches of the Whomping Willow were flailing. They spot the knot that freezes up the tree.
“I can see the place,” Ron says. “If we just had Crookshanks again —”
“Crookshanks?” Hermione replies. “Are you a wizard, or what?”
This was a reference back to the Philosopher’s Stone when Hermione had to conjure up fire to free the other two from the Devil’s Snare. She says that she does not have any wood to light the fire.
“HAVE YOU GONE MAD?” Ron yelled at her. “ARE YOU A WITCH OR NOT?”