[Houmatt]

It was obvious to me from reading the things Slughorn said he bungled names frequently.

But the Rupert "slip?" It was not. By this time, people are associating Rupert Grint with Ronald Weasley. Jo knows this, so it was done with a knowing wink to the reader (and perhaps Rupert, too).

I'd bank on that.

[WV Mountain Mama]

I thought it was a joke on her part too, pretty funny. I too, was not enlightened about who the half blood prince was.