It could be author's error. In fact it probably originates as author error.
But how much smarter than Eliezer do you think Harry is intended to be?
In order for something to be an idiot ball it has to be significantly stupider than the character is supposed to be. This mistake is not significantly below the threshold for Harry; it's actually less stupid than his decision to go with Quirrel to Azkaban. Or his impulsive decision to boost his patronus up to maximum. Or his failure to consider the difference between flying a rocket and flying a broom.
Calling any failure in perfect rationality an "idiot ball" is accurate only when dealing with a perfectly rational AI.
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.
You can't improve the result by injecting randomness. This is a normative argument, which is distinct from a factual observation that the author can't avoid errors. Even if in fact the author can't avoid all errors, it's still preferable to avoid errors where possible.
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: