'I did think he might be a bit better this year,' said Hermione in a disappointed voice. 'I mean… you know…' she looked around carefully; there were half a dozen empty seats on either side of them and nobody was passing the table '… now he's in the Order and everything.'
'Poisonous toadstools don't change their spots,' said Ron sagely. 'Anyway I've always thought Dumbledore was cracked to trust Snape. Where's the evidence he ever really stopped working for You-Know-Who?'
“Poisonous toadstools don’t change their spots” is a wizarding phrase meaning “people can’t change who they really are inside.” Ron quotes this sage advice when Hermione says she thought that maybe Snape would be less mean to Harry in their fifth year since the Potions Master was in the Order (OP13).
The phrase is a wizarding version of the common Muggle phrase "A leopard can't change his spots." The phrase comes from the Old Testament of the Bible: “Can the Ethiopian change his skin, or the leopard his spots?” (Jeremiah 13:23).
From the Web
"a leopard cannot change its spots" on Wiktionary