A post-credits plot twist is a story event that takes place either after or during the credits, either in cutscene or even a simple audio conversation form. The scene relays information that serves to either deliver a cliffhanger or plot twist, regardless of how neatly the narrative wrapped up prior to the credits. As a storytelling mechanism, when invoked, it is often used as set-up for a potential sequel.
Metal Gear Solid Series
The Metal Gear Solid series makes frequent use of post-credits sequences to deliver vital story information. For example, in Metal Gear Solid 2: Sons of Liberty, Snake and Otacon acquire a list of names that they presume to be the members of the secretive organization known as the Patriots. However, in a conversation with Snake after the credits, Otacon informs him that the list they acquired was a red herring; all of the people on it have been dead for over a century.
In Metal Gear Solid 4: Guns of the Patriots, there is a lengthy post-credits sequence in which Snake, in a cemetery, is met by none other than Big Boss, accompanying a senile and vegetative Zero. Big Boss explains to Snake how Zero had not only founded the Patriots along with their former allies, but how his desires grew corrupt, leading to the Patriot A.I. system and all of the troubles they caused. Big Boss then cuts off Zero's oxygen tank, killing him, as everything would end with Zero, just as it had begun. Big Boss subsequently passes away himself, as he was infected by the FOXDIE virus that Snake still carries.
If the player is able to finish Metroid Prime with 100% completion, a special sequence after the credits depicts a hand rising out of a pool of Phazon. This scene is the birth of Dark Samus, who goes on to become the primary antagonist of Metroid Prime 2 and 3.