A good opening book will discuss move order finesses. It's important to know that in a certain variation it's better to play A first and only then B, because in that way you cut out certain of the opponent's options. Well and good, but what about this, from Victor Bologan's new book on the King's Indian?
[After the moves 1.d4 Nf6 2.c4 g6 3.Nc3 Bg7 4.e4 d6 5.Be2 0-0 6.Bg5 Na6 7.Qc2, Bologan writes]
7...h6
White's position is slightly better after 7...c5 8.d5 h6 9.Bf4.
8.Be3
Or 8.Bf4 c5 9.d5 [further analysis ensues, all of which is supposed to be fine for Black]
Of course, the position is exactly the same in the two cases. Both times Black plays h6 and c5, and both times White plays Bf4 and d5. But if Black plays ...h6 first, then White is better, and if he plays ...c5 first, then White's not! It's for subtleties like this that I get opening books, and Bologan's ability to suss out such fine points makes it clear what a vast gulf there is between super-GMs and the rest of us.
(Seriously, though: this gaffe notwithstanding, I'm a fan of Bologan's writings in general and the book in particular. But I do wonder what he was thinking when he wrote that.)