by redshoesrock » Tuesday 19 July 2005 6:55:12am
It wasn't meant to be a typo - it's supposed to show that Slughorn's mind was on other matters, namely getting Aragog's venom. It also is meant to show that Ron didn't rate very highly with Slughorn, as compared to say, Harry. Though I wouldn't be surprised if JKR chose "Rupert" for Slughorn's slip-of-the-tounge because of Rupert Grint, the actor that plays Ron Weasley.