Hey, careful with the spoilers, there! Some of us haven't yet received the book from our independent online retailer of choice!

Hey, at least you don’t have to live with someone who’s constantly taunting, “Who do you think the Half-Blood Prince is?”

She finished the book at about eight o’clock — twenty-eight hours or so. We didn’t let her read it during church, though she did read at dinner (by special permission).

This isn't an error. You'll recall that the plotline features the fact that Ron isn't one of Slughorn's "favorites." Clearly, here he has forgotten Ron's name. Though, yes, Rupert was not likely a random choice on the part of the author, even if it was for the character. Good catch, though Pippa! It made me wonder, too.

Check out my thoughts on the book at my blog.

Yeah, I'm with Micah on this: I think the choice of name may be a subtle reference to the movie(s). Unlikely someone in the editing process didn't catch it. But yes, I think Micah has it right: I got to that part last night and immediately thought of Slughorn's selection criteria.

On a side note, 31,000 copies were delivered here in Seattle on Saturday: I wonder how many are completed?

Earlier in the book (I think when something nearly catastrophic happens to Ron--tried not to make that a spoiler...) he calls him by the wrong name too. (Robert? Rufus?) So, though I do think it's a reference to the movies, it is also the continuation of the "Ron's not important enough to notice/remember" plot line.

I'm not so sure... remember, when Goblet of Fire came out, there was an editing error in the final battle scene, where some of the first printings of the book reversed the order in which Harry's parents died.

Sorry to chime in so late - I've been ignoring all Harry Potter posts until I finished the book, which I did tonight.

I'm glad to see that someone else noticed this, I thought it was odd immediatly and was trying to figure it out. But I agree that it is part of the "Ron's not important" plotline, because as mentioned above, on pg 397, Slughorn calls Ron "Ralph" and Harry tries to correct him. Thanks for pointing that out, it would have driven me nuts.

My resident Potterite comments:
thats not an error haha as one of the people who comment mention it is because he is simply not one of that teachers favourites and that teacher only bothers to learn the names of certain characters who are famous or have connections and ron doesnt hahah not a mistake!!