If there should be an error, this should be decided by the author, not chance. Having a character make an error where they shouldn't is equally an author's error as having a character not make an error where they should.
And this is a situation where it's reasonable either way. Noting that Eliezer has a tendency to err on the side of making his characters too rational, except where necessary for the plot, rather than the other way around, I can guess that Eliezer probably intended that Harry not make an error.
However, Harry making an error here actually increases the verissimilitude to me; now that I know about it. It makes the plot-relevant errors seem less "idiot ball"* and more "Harry makes errors occasionally, whether the plot requires it or not"
*(in the sense of "this person needs to make a mistake in this scene for the story to go the way I want")
You can't improve the result by injecting randomness.
You can if the alternative is worse than random (ie. a character only making mistakes when the plot requires it)*
*and the only reason I believe this was a random mistake is that I suspect Eliezer of having that flaw. So, if Eliezer lacks that flaw, I expect that this was intentional.
Is it an error? The reply in McGonagall's voice with Dumbledore's voiced reaction is good authentication, he is after all still supposed to be under Quirrell's guard, and trying to avoid causing suspicion.
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: