It was unlikely that his enemies would see what was being done, so unlikely that they did not think to prepare. So unlikely that, appropriately enough, it was mentioned in chapter 78:
The Dragons had started the combat with a feint to provide a distraction for Mr. Goyle's approach through the forest; Neville hadn't realized there were two brooms attacking until almost too late. But the Chaos Legion had gotten the other pilot. That was why broomsticks usually didn't attack before armies met, it meant a whole army would concentrate fire on the broomstick.
The Dragons sacrificed a broom to see what was up. This is a significant sacrifice as they have only two and their enemies have a total of four between them.
It would not be surprising for the next battle, if there is one, to include some protection against aerial espionage. Better spy-planes would be another reasonable result of this battle. The first thing you do is fly higher.
First of all. Thank you for pointing this quote out!
Well, I'd say the first thing you do is hide better :) It hardly needed much more than a cloack over the object about to be transfigured. Perhaps another weakness of Harry's, overcomplicating? I appreciate the sacrifice made and the development by the character of Draco. Still, for any good mastermind in the making keeping your top-secret plan top-secret is usually an extremely good idea.
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
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: