Poisonous toadstools don't change their spots.

This is your copy of Advanced Potion-Making, is it, Potter?”
“Yes,” said Harry, still breathing hard.
“You’re quite sure of that, are you, Potter?”
“Yes,” said Harry, with a touch more defiance.
“This is the copy of Advanced Potion-Making that you purchased from Flourish and Blotts?”
“Yes,” said Harry firmly.
“Then why,” asked Snape, “does it have the name ‘Roonil Wazlib’ written inside the front cover?”
Harry’s heart missed a beat. “That’s my nickname,” he said.

Snape'in Đksir dersinde kasıtlı olarak karmaşa yaratmak,
uyuyan bir ejderhanın gözüne parmak sokmak kadar güvenliydi.

Snape'in İksir dersinde kasıtlı olarak karmaşa yaratmak,
uyuyan bir ejderhanın gözüne parmak sokmak kadar güvenliydi.

Blimey! There are two of them!

An Unbreakable Vow?" said Ron, looking stunned. "Nah, he can’t have.... Are you sure?"
"Yes I’m sure," said Harry. "Why, what does it mean?"
"Well, you can’t break an Unbreakable Vow..."
"I’d worked that much out for myself, funnily enough.