In an old 1E campaign over 20 years ago, the PC's had been captured and released into a large cavern maze sans most equipment to be hunted down by an evil "master of the hunt" overlord for his amusement. The party ran, hearing the baying of the hounds getting closer and happened to run into a passage that dropped off into a seemingly bottomless chasm. The other side of the passage was over 50 feet away. There was a "bridge" of sorts made up of webbing from a horrendous huge spider that was nowhere to be seen.
The party started making thier way across the web climbing over the dried husks of dead giant insects. The hounds were getting closer and a little over halfway across the spider shows up. Moving slowly in the web and having few weapons the situation looked grim. The druid decided to throw caution to the wind and unleashed blue fire at the spider. The damage was enough to do the job but it also burned through the webbing serving as a bridge!
The party was now falling down a huge chasm and I had nothing.
The rest of the session was 100% pulled out of you know where.
........Of course there was deep water below, a giant underground lake. The party turned the insect husks into canoes and followed a river leading out of the lake.
The river eventually led out the side of a mountain far to the South in unfamiliar lands. The climate and plant life was tropical and nobody (including me!) had any idea where they were. I was able to convey the confusion and strangeness of the surroundings well because it was as bizzarre to me as it was to the players.
Below the mountain was dense jungle. The party began to explore as I raced to create the space in front of them. The party discovered a strange mechanical flying contraption that had apparently crashed up in a tree. Inside were the remains of several strangely dressed humanoids and a couple of weird artifacts. ( A flashlight and a loaded .45 ACP). The party had unknowingly stumbled onto the set of
Predator.
The roleplaying with the artifacts was great. One of the fighters claimed the flashlight convinced it was weapon of awesome power.
The PC's soon encountered a tribe of natives who were living in fear of the ghostly
Ilguu. The party decided to befriend the tribesmen and hunt down thier problem.
It was an epic fight, PC's with primitive bone armor and flint weapons against the demon who made trophies of men. On the first round of combat the fighter with the flashlight shined it right in creature's face expecting it to be vaporized.

The powers of the .45 were discovered and the PC's returned with the head of the creature.
End session.

Good times.
Before the next game I created a whole ruined city nearby to explore filled with strange homebrew creatures and items.