When good plans go awry

I was playing a session of D&D last night with my group. They were just about out of resources for the day and had holed up to sleep. It seemed like a good time to taunt them by having the BBEG (big bad evil guy) show up at their doorstep and then leave. But they had decided that they'd had enough. They popped out of their rope trick and blasted him. One critical hit with a disintegrate ray, a cruddy initiative roll on his part, and a few clobberings later, the BBEG was dead and the players were celebrating. Curse them and their infernal competence.