Here's an easy recipe for hilarity: take a handful of gamers, add some rulebooks and lorebooks, shake in some dice (optional), and add a Big-Lipped Alligator Moment or two. Season to taste.
- Deadlands is a Wild West setting where all the ghost stories are true. The first huge adventure in the Classic era was the three-part Devil's Tower trilogy that directly ties in to the sequel setting Deadlands: Hell on Earth. After the posse goes toe to toe with some of the most dangerous bounty hunters in the "City o' Gloom" and learns the horrifying truths of what goes on in Reverend Grimme's island prison, the posse makes it to the eponymous Devil's Tower and discover that it's inhabited by aliens from outer space. Suffice it to say, this was never mentioned again in future modules, and when the area was revisited in Deadlands Reloaded: The Last Sons, that plot point was very much downplayed.
- One of the earliest Dungeons & Dragons modules, S3: Expedition to the Barrier Peaks involves the characters being sent into a cave and discovering that it's actually the remains of a downed spaceship! This event is never alluded to or referenced in Greyhawk's long history following this module, despite the societal impact it's bound to have (beyond a few monsters that debuted in the module having references to possibly being from another world in their Monster Manual writ-ups). Somewhat justified, as only adventurers would get out alive, and most D&D players would be more interested in using the miraculous discoveries to complete quests rather than improve society.
- The classic AD&D module "I6: Ravenloft" is a thrilling, wonderfully-planned and -mapped Gothic vampire hunt, full of genuine scares and tributes to old Universal and Hammer horror films. It plays it out as straight as can be ... until you get to the underground crypts, and realize that every tomb's engraving is some Incredibly Lame Pun that wouldn't amuse a six-year-old. Definitely a this-didn't-happen for fans and designers of the later Ravenloft setting; when the module was updated and re-released as "Expedition to Castle Ravenloft", they deliberately expunged this Old Shame, making the crypts' inscriptions illegible or pun-free. The 5e remake Curse of Strahd brought it back, with lots of updated jokes.
- Magic: The Gathering has a joke cardset called Unglued that cause BLAMs. One card turns an opponent's creature into a chicken and another causes your opponent to play with one hand behind their back. These cards are not allowed in tournament play.
- Random Encounters in general in games can be this, as they are by definition random and don't have any backstory behind them. Particularly if whatever creatures you encounter are both sentient and don't appear anywhere else in the module/dungeon, which raises the obvious question: What the hell were they doing there in the first place?