Then there are a number plot holes wrapped into one confounding moment: when Dumbledore looks into the Mirror Of Erised and sees a flashback of himself and Grindelwald making a promise to never fight each other. First off, the Mirror of Erised doesn't show flashbacks, a pensieve does, and Dumbledore used one in the Harry Potter films. (Thus, he knows how to use it!) The Mirror of Erised shows the user's deepest desire, and we already know that Dumbledore's is his happy, healthy family, removed from the tragic events with his sister, because this was explained in the books.