A couple of years ago at GenCon, I ran a Buffy the Vampire Slayer game called 'Dungeons and Bunnies"
The game started with the original cast gathered for Anya's birthday party. Anya was an ex-demon, who had spent hundreds of years granting vengeful wishes to wronged women. One of her past victims breaks into the party and casts a spell, trapping Anya in an alternate dimension. This particular spell creates a prison for the victim, based on their fears and whatever else is in the area.
Anya is afraid of bunnies, and the beam hits Xander's shelf of D&D books.
The PCs find themselves on a road in the middle of a forest. Buffy has a magic looking shield and sword. Willow was a sorceress, Tara a druid, Giles a wizard, Xander a thief, and so on. They wander into the village. The blacksmith and fishmonger greet the PCs warmly, but respond to any quesion with 'you should ask at the Inn!'. When they ask for their names the villagers all say 'I guess I don't have one!'
The Colorful Monster Inn is a brightly lit place, except for shadowy corners. Each corner had men with cloaks pulled low. Everyone else is the bar was an old man holding a treasure map, offering them to passing adventurers. Any one of them could tell the PCs that their friend was no doubt being held prisoner at the Fane of the Mad Bunny God, and here is a map they can sell you.
It was then that the realized it. Not only were they in a D&D adventure module, they were in a really crappy one.
On the way they saved a broken down wagon from bandits, though the merchants couldn't say where they were from or where they were going, they had always been broken down on the road. They were so grateful to the PCs for saving their 50 sp worth of grain they gave them healing potions worth several hundred gold.
More jokes inisde - a killer rabbit with big sharp pointy teeth at the cave entrance, a floor puzzle that one must spell Gygax, and the Dread Gazeebo. My favorite scene in the whole game - the group splits, with Xander and Giles trying to get past a runed door, the rest exploring a small courtyard.
Me: You see a small courtyard, overgrown with ferns and mosses. In the center is a gazeebo.
Buffy: Huh. Why's there a gazeebo in the middle of the dungeon?
Xander(hearing from down the hall): Oh god - RUNNNN
The gazeebo, naturally, attacked 4 times a round, had 150 hit points, and was completely immune to arrows.
Everyone had a blast and seemed to enjoy the 4th wall breaking nature of the game - characters realizing that they're in a FRPG adventure module, and not one of the good ones.