This is funny, as in my career I've seen a number of domains, and built (web) systems which required a form of undo. You just don't see it too often in the PHP

This is funny, as in my career I've seen a number of domains, and built (web) systems which required a form of undo. You just don't see it too often in the PHP/Ruby crowd because it's hard to get right and each implementation has inherent limitations. Luckily there has been work on patterns since the CoF Command/Memento: the Event Sourcing pattern (http://martinfowler.com/eaaDev/EventSourcing.html http://martinfowler.com/eaaDev/EventSourcing.html) can be used to implement system-wide undo functionality.