Am I losing my mind, or was there a change made to Chap 16? I recall this section:

" No, there is exactly one monster which can threaten you once you are fully grown. The single most dangerous monster in all the world, so dangerous that nothing else comes close. The adult wizard. That is the only thing that will still be able to threaten you."

However now it reads:

" No, there is exactly one monster which can threaten you once you are fully grown. The single most dangerous monster in all the world, so dangerous that nothing else comes close. The Dark Wizard. That is the only thing that will still be able to threaten you."

If it was changed... why the change? The original was better, and (perhaps more to the point) more in keeping with Quirrell's character. He wouldn't distinguish between adult and Dark wizards when it comes to threat-to-his-students assessment.