Detention With Dolores
At dinner that night, Harry hears people muttering about his scene in Defense Against the Dark Arts.
Hermione finally levels with Harry: "It's just that before the truth [about the return of Voldemort] could sink in [at the end of the last school year], everyone went home for the summer, where they spent two months reading about how you're a nutcase and Dumbledore's going senile!" (13.13).
As they settle in to do homework in the Gryffindor common room, Hermione sees Fred and George up to something.
They're testing their Skiving Snackboxes on a bunch of first years.
Hermione shrieks, "I told you this morning, you can't test your rubbish on students!" (13.34).