No matter how well you plan and prepare your adventure or campaign, something will always come along which will throw your plans straight into the trash. Your player characters fight when you plan for them to run away; or they run away from a combat scene. You set them up for their first skirmish with the scenario's Big Bad, and somehow they manage to kill him; or you roll for a wilderness encounter, and a tiny party of kobolds somehow make critical successes and wipe out the party on their very