One of the hardest things for me to do as a GM has been to let go of the idea of the maps in my mind.
The simple example is that I imagine the boss is on the left tower, but the players go to the right tower. As a GM, one needs to be flexible enough to put the boss in the right tower.
More complex are deep plot points which I imagined might come out a certain way, but there's no reason they can't come out through an entirely different interaction or encounter.