A joke ending is an optional, alternate ending that is meant to be entirely humorous. Such endings, particularly when appended to games with otherwise serious or dramatic storylines, are more often than not meant as lighthearted, absurd bonuses for players that have already completed a particular game at least once.
The Silent Hill series is notorious for the inclusion of joke endings dating back to the original game. In the midst of searching for his daughter, Harry Mason encounters aliens and asks them if they've seen her. They respond by abducting him. In Silent Hill 2, James Sunderland can come across a secret room in which a dog is stationed at a control panel, and it's revealed that all of the town's horrors were the doings of the canine. In Silent Hill 3, Heather Mason returns home to her father, who is alive and hosting an alien as a house guest.
The game Metal Saga has numerous alternate endings. The easiest to find can actually be triggered during the very first conversation in the game. If the player character acquiesces to his mother's request and chooses to stay home and become a mechanic, the game will go on to describe the many absurd adventures that the player character could have had, had he not decided to live the boring life of a mechanic. After this, the credits roll and the game ends.
In Shadow Complex, there exists a point where it is possible to actually jump a gap that will lead the protagonist Jason Fleming back to the jeep that he and Claire, the game's damsel-in-distress figure, arrived in. Jason will actually climb in the jeep and drive off, leaving Claire in peril as he ponders that there are other fish in the sea.
Persona 4 Arena
In Persona 4 Arena, the story mode contains joke endings fro three of the game's characters; Yukiko Amagi, Chie Satonaka, and Kanji Tatsumi. The player can view these endings by simply selecting the proper option when given a choice of paths to take. In Yukiko's case, she will feed her ruined boxed lunch to Teddie, whose immediate, violently ill reaction is mistaken by her for a sign that it has energizing properties. She then goes on to feed it to the rest of her friends, not realizing the next day that the illness they all suffer from was brought on by her disastrous cooking. In Chie's, Chie follows Akihiko Sanada through a window that takes her to a convenience store. She and Akihiko then fight over the last remaining beef bowl. Just as Chie is celebrating her victory, however, she awakens in the real world and discovers, as she is being scorned by her friends for prioritizing a beef bowl over them, that she had been duped by an illusion. In Kanji's joke ending, Kanji continues on in the P-1 Grand Prix, unaware that he isn't dreaming. At one point, he comes across Naoto Shirogane, and when she addresses him formally, he insists that she refer to him casually as just "Kanji." Later, after Kanji finally understands that he's actually in the TV world and not dreaming, he is confronted once again with Naoto, who had taken his earlier insistence as a sign of romantic interest. Realizing what he's gotten himself into, he panics.
In Jordan Mechner's original Karateka, the player's life depletes to almost nothing upon defeating Akuma, the final boss. Princess Mariko, the damsel-in-distress, awaits in the final room. However, if the player approaches Mariko at walking speed (the fighting stance), Mariko will kick the player character in the head, killing him instantly. In order to truly win the game, the player must run to her at full speed.