Hard to say, because we don't know all the details of his plans going on in the background, and a lot of what we do know is dependent on a maybe-real near-death experience. But I can see at least three major contenders.
1. Leaving Harry with the Dursleys without providing more oversight into his treatment/living conditions. Harry turned out remarkably well-adjusted, I suspect, for a chronically abused and neglected child. Even leaving aside the obvious moral considerations, Dumbledore got really lucky that that didn't fuck up his strategy.
2. Trying to pass the Elder Wand off to Snape. I don't know, but I doubt it would have counted for Snape to kill him when Dumbledore let him do it. Seems an obvious blunder.
3. Not providing better self-defence training to Harry. Even if he was never intended to duel Voldemort to the finish, he had to survive long enough to ensure the rest of the plan worked out, and since the plan didn't actually depend
on Harry dying (at least past book four), but just being willing
to sacrifice himself, giving him the best chance possible would be the right thing to do.
Elheru Aran wrote:My only reaction to that is 'ew'.
Yeah, I don't think Voldemort fucking is an image anyone needs.
Anyway. Point is, the HP books are pretty shit at setting up background for Wizarding Britain. Society, culture, what have you-- the books are the story of a near-Muggle being more or less forcibly thrust into an almost completely foreign environment that is atypical even for itself-- he's in a school which isn't the same as the 'outside world', said outside world is affected powerfully by the recent terrorist acts of Voldemort to the point that they're afraid of even saying his name after they believe he's dead, etc. And with the books being our only example, apart from the movies which aren't much help in themselves, we simply can't say much with certainty one way or the other.
This is part of what I enjoy, and am looking forward to seeing more of, in the Fantastic Beasts movies- more of an opportunity for world-building, in a more entertaining format than looking up and reading articles on Pottermore.
"Our progress in degeneracy appears to me to be pretty rapid. As a nation, we began by declaring that "all men are created equal." We now practically read it "all men are created equal, except negroes" When the Know-Nothings get control, it will read "all men are created equal, except negroes, and foreigners, and Catholics." When it comes to this I should prefer emigrating to some country where they make no pretence of loving liberty -- to Russia, for instance, where despotism can be taken pure, and without the base alloy of hypocracy." - Lincoln.