The passage of interest:
"I cared about you too much," said Dumbledore simply. "I cared more for your happiness than your knowing the truth, more for your peace of my mind than my plan, more for your life than the lives that might be lost if the plan failed...What did I care if numbers of nameless and faceless people and creatures were slaughtered in the vague future, if in the here and now you were alive, and well, and happy?" (OOTP, US Hardcover, 838-839).
The above passage is Dumbledore's explanation of "The Flaw in the Plan." It is spoken, and then followed up by Dumbledore telling Harry about the prophecy, conveying to the reader--and more importantly, to Harry--that the "flaw" was not telling Harry about the prophecy earlier, presumably so that he could start his horcrux hunt sooner and bring about LV's demise sooner.
- Is this specifically the way the future slaughter of people and creatures would be avoided? I think not.
Might Dumbledore have been referring to a much bigger flaw? One that he didn't mention to Harry in this room, but one that he cryptically began to introduce here?
I speak of the plan for Harry to sacrifice himself so that LV will never be able to hurt anyone again. Per:
"You won’t be killing anyone else tonight,” said Harry as they circled, and stared into each other’s eyes, green into red. “You won’t be able to kill any of them ever again. Don’t you get it? I was ready to die to stop you from hurting these people—” (DH, US Hardcover, 738).
- There is a really important takeaway from the above passage: Even if Harry had proven unable to destroy a single horcrux, his sacrifice would have prevented LV from hurting any innocent person ever again.
From these two quotes, I glean the following: The "plan" was Harry's self-sacrifice. If this plan failed, the people would not have been protected against LV. The "flaw" was not necessarily Dumbledore's resistance to tell Harry about the prophecy/horcruxes, but his overvaluation of Harry's life when it would have been far more important for Harry to just sacrifice himself (to shield against future harm for "numbers of nameless and faceless people").
Of course, Dumbledore COULD NOT have been truthful here about the plan, otherwise the sacrifice aspect would have been negated. It is nonetheless interesting that Dumbledore tells Harry he is ready for the truth, and then proceeds to lie again, even if it was necessary to start moving Harry in the path toward his sacrifice.
Having reached the end of this long-winded background, I ask the following: What did DD mean specifically by "the Flaw in the Plan" (Horcrux or Sacrifice)?