I was the player who derailed the campaign.
The DM had us working our way through some subterranean caverns, rooting out cultists, and searching for some rumored monster who guards the cave. My Paladin was delivering Bahamut’s justice right and left, and as we made our way deeper into the cavern, we noticed that the lower we explored, the wetter things got. Soon we were jumping over fetid pools and stinking streams in the lowest levels of the dungeon.
When we penetrated the final room, what we presumed to be the the lair of this mysterious monster, we found an alter on the opposite side of a large cavern, with a foul-smelling underground lake between us and it.
Quick digression: did I mention I was playing a Paladin? He was heavily armored, as Paladins tend to be, and more than any monster, he was terrified of drowning, being dragged under water in his heavy armor, and unable to surface.
Assuming we were going to destroy the alter and battle whatever monster emerged from the shadows when we did, my character used the Pouch of Frozen Passage to freeze the underground lake solid, so our party could walk across the water in safety. I had it in my inventory, purchased legitimately in-game, and used it according to the DM’s discretion.
I had no idea that the scheduled boss monster was some kind of aquatic creature who was planned to emerge from the lake and attack us… the lake which was now frozen solid.
After multiple failed attempts to damage the ice from beneath, the DM announced that our party was unable to locate the monster rumored to dwell in the depths of this cavern, so we destroyed the cultists’ alter, and wandered casually out of the dungeon.
He obviously could have retconned his plan and had it emerge from some hidden tunnel, not out of the water, but he wanted to play it completely straight, and he figured it would be funnier if we never located the beast.