In my first run of Final Fantasy XII back on the Playstation 2, I didn't use the manual.
I was trying to max everything on all the characters. And because the license board was the same for every character, it meant every character could do everything (I forget if there was a limit on unlocking weapons, because it's been so long).
Anyways, to get the strongest weapon in the game, there was an treasure chest that had nothing special about it that wasn't supposed to be opened. Of course, being the min-maxer I am, I go to every corner of every map and open everything. Bad idea. Just opening that treasure chest which seemed to have no significance killed my dream of having the best weapon. Sometimes the game will ask if you are sure you want to do something and you second guess it because not doing the action may lead to something better later, but this didn't happen here.
So when I got to the main end area to beat the game, I read the strategy to find why I hadn't unlocked that weapon. Had to go all the way back to the beginning and re-level/replay everything up to that point to unlock the weapon.