The last two sessions of my Mutant Crawl Classics campaign were improvised. I didn't have anything published I was running. The first time I had a plan. The PCs got a broadcast from up north that a military base was available to be looted and the PCs wanted part of that. They head up north, got in a fight with a road gang, and invaded the base. And they made a giant robot mad. Worked pretty well.
But the last one didn't work so well. The giant robot was chasing them so they headed off to find some robotics experts. That turned into a herd of mutant elf trampling their car and cyborgs who took them off in a spaceship to the inside of a volcano where another giant robot was being built.
It got crazy and was less and less players making decisions about what cool things their characters were doing and more of me the GM making up crazy stuff. I went off the rails and it wasn't good.
So now I'm taking a published adventure and looking through it, seeing what I want to use, and preparing a solid structure for next session.
But the question I want to ask you all is: Do you ever go off the rails like that? How do you rein yourself in?