As much as I had fun speculating about how this might play into canon, and hoping that it wasn't just an oversight, part of me knew, deep down, that ( Spoilers for latest update on jkrowling.com )
Nevertheless, there are a lot of people out there who are disillusioned with JKR right now -- something Jo herself obviously anticipated, given the wry, apologetic tone of her answer. The question I want to ask is, are our expectations of JKR reasonable?
It's easy for us as fans to pick up on minute details of the Potter universe (though we argue and disagree even about those, so the answers aren't always as obvious as they may seem), because all we have to do is read what JKR's written and think about it, and many of us have ample spare time in which to do both. We aren't producing
the books, feeling the pressure of millions clamouring for the next one, trying to work as quickly as possible yet maintain some level of quality, and at the same time trying to juggle writing with family and social life. The wonder to me isn't that JKR makes mistakes from time to time; it's that she hasn't made even bigger ones.( Well, yeah, but surely a responsible author would... )
Even if Jo could somehow miraculously live up to her readers' expectations and get all the dates, moon phases, character names, mathematical calculations, and other details right in an epic seven-book series that covers thousands of pages -- if every loose end were tied up and every question answered -- what would we fans have to talk about? All right, that's a bit facetious, but -- there is a unique satisfaction in coming up with a neat and well-reasoned explanation for something that would otherwise appear to be an obvious inconsistency or mistake.
The best example of this is in Sherlockian fandom, where (as in HP) you're dealing with a single author, a large and complex canon, and millions of obsessed fans. In fact, there are few characters in literature whose popularity and worldwide recognition compares with that of Sherlock Holmes -- only time will tell if Harry Potter can hold onto the same kind of status.( What do Sherlockians do? )
In JKR's case, all the evidence seems to indicate that she loves her characters, she's wholly committed to the story she wants to tell, and she's doing the best she can under very high-pressure and distracting (believe me, if you have a preschooler, you know about distracting) circumstances. Yes, she has made mistakes, and will doubtless make a few more along the way. But considering the amount of pleasure she's brought us, and the sheer scope of the story she's telling, I think that we fans can afford to cut her some slack.
That doesn't mean ignoring the errors. It doesn't even mean refusing to admit that they are
errors (although, as I said above, denial can be fun). But I don't think anything good is likely to be accomplished by grumbling among ourselves about how JKR's a careless writer and if we
were writing the series we'd do a better job yadda yadda. Especially since the former claim really isn't true and, in 99.9% of cases, the latter isn't either.*
*I will really look forward to seeing the fabulous original epic fantasy novels produced by the remaining 0.1%, though...