In one campaign, the first quest was a fetch quest into a random cave. In the cave, having found the item, they were accosted by a small stone which demanded to be skipped. One of them did. They were teleported to a Chaotic plane. There the campaign became something between a series of bad jokes and survival horror.
I keep trying to force them to think outside the box, though most of the time they just ran. In the end they meet back up with the stone (now a giant whirlwind dervish monster) who they fought. They got the idea that by killing themselves, they would defeat the monster.
It was not what I had planned, but I went with it, because its just great when the whole party commits seppuku to defeat the BBEG.