d20 Despot: Expect the Unexpected
One of the first things you learn as a GM is that the players will absolutely wreck all of your plans at the first opportunity. You can never make a plan that takes into account every possible action by your players. This is part of the beauty of tabletop roleplaying games.
Expect the Unexpected
How do you prepare for a game when you know your players are just going to wreck all your plans? Expect the unexpected.