See, this shit wouldn't throw me. As a DM I love improve in my plot lines. Hell, I often don't plan things out past laying out the world and enemy motivations.
Sure my big bad has goals, but they have to work for them just as much as anyone else does. One of the most fun (for me) campaigns I ever ran was where the big bad conducted a ritual off screen, and botched the roll. He ended up killing himself.
The party knew that he had been planning a ritual of some sort and had gone into seclusion. They had decided to dismantle his organization before tracking him down.
So weeks of fighting and taking out little hidden cells of the cult, they finally find info about where the big bad was doing his ritual. Only to find a mangled rotting corpse and a closed hell portal.