Actions whose effect on the game state cannot be undone.
All events in a game affect the game state but many may be countered by another events, e.g. may a movement in one direction be countered by a movement in the opposite direction. Game state changes that cannot be revert back are the effects of Irreversible Events.
Pushing a box into a corner in Sokoban is an Irreversible Event since the box cannot be moved from there, as the boxes can only be pushed and not pulled.
The Ko (and meta Ko) rule in Go is used to make it impossible for the game to run into infinite loops and without these some events in the game would be reversible.
Using the pattern
Making events into Irreversible Events can be done by simply making sure that all other events in the whole game design do not affect a specific part of the game state (other events may affect that part of the game state as long as all affect it in the same way or direction). Other ways of making Irreversible Events are to make actions use Non-Renewable Resources (in which case the action rather that the consequences of the action may be Irreversible Events) or create events that give players information. Designed Surprises are thus a form of Irreversible Actions, and since they are a form of Trans-Game Information, they are even Irreversible Actions across game sessions. Events that provide Strategic Knowledge are likewise Irreversible Events since these also generate Trans-Game Information.
The possibility of Saving makes it impossible to have totally Irreversible Events in a game if players are willing to make use of Save-Load Cycles. This can be countered to a certain extent by causing actions to have Delayed Effects (c.f. The Witcher) at the cost of Predictable Consequences.
Buttons and Alarms are game elements that can be used to enforce Irreversible Events, in the first case by not being able to be activated more than once and in the second case by not being able to be deactivated once activated (or vice versa). A typical example of a class of Irreversible Events is Leaps of Faith. Making the Transfer of Control of Resources into Irreversible Events is part of creating Non-Renewable Resources.
Irreversible Events can created to be the consequences of actions but the actions themselves do not have to be irreversible their effect may be since partly or completely prevented if they are Interruptible Actions or Extended Actions. Likewise, actions that cause Ultra-Powerful Events do not have to be Irreversible Events since other Ultra-Powerful Events may counter the event after it is completed. An example of this is Shrinking Game Worlds: these are usually Ultra-Powerful Events that cannot be affected while they are occurring but after the Game World has become smaller it may at a later stage expand again.
Every Irreversible Event that a player understand to be irreversible is a closure. Although this can indicate progress or failure clearly, the presence of the pattern may negatively affect Experimenting since there is no Reversability, and make Puzzle Solving more difficult. Rather, the presence of Irreversible Events promote Stimulated Planning and may cause Analysis Paralysis.
Irreversible Events are required to progress Narrative Structures and can be used to provide a condensed history of gameplay in a game session without losing any events that affected the final outcome. Leaving a Level and not being able to come back to it is an example of a Narrative Structure can be maintained by Irreversible Events. An example of how Irreversible Events can advance the Narrative Structure in Real-Time Games without relying on Levels is to move the places where Spawning occurs.
Potentially Conflicting With
An rewrite of the original pattern named Irreversible Actions in the book 'Patterns in Game Design' (Björk & Holopainen, 2004).
Björk, S. & Holopainen, J. (2004) Patterns in Game Design. Charles River Media. ISBN1-58450-354-8.