> This may be terribly crass. If it is, please forgive me, only this has been 
> brought up enough times for me to begin to wonder whether this just *could* 
> be one of those English usage things. I'm sure I'm stating the obvious, but, 
> in British usage at least, 'Ah, you've noticed the deliberate error,' is such 
> a common way of admitting a mistake (perhaps JKR's, rather than the editor's, 
> hence the tongue-in-cheek response?) that I've always taken it to be just 
> that. As Judy so graphically demonstrates, you have to jump through a lot of 
> rings to get 'ancestor' to make any sense.
> 
> Eloise,
> glad that Judy liked her *brave* Riddle interpretation.

I actually never knew about the "deliberate error" British euphemism but I thought that maybe by "deliberate error" she could have meant (not necessarily that she did but that she could) that she put ancestor instead of descendant just to see if people were paying attention, which, obviously they were. Perhaps overly so at times.
BTW, I'm usually a lurker here. Due in large part that I have a hard time keeping up with so many postings and I try not to get into a discussion that I haven't paid attention to from the beginning. But I probably will add in my 2 pence occasionally so just to let you know my name is alley and I'm an undergraduate currently in the noertheast part of the US.
Alley