Agreed. It becomes especially complex in a distributed system with multiple users having access to the same data. For instance, even a simple message exchange,

Agreed. It becomes especially complex in a distributed system with multiple users having access to the same data. For instance, even a simple message exchange, which has no data interdependency issues, an undo feature requires some kind of design trade-off. Either the application lies to the user and delays the sending of the message to allow for an undo period, or it allows the message to be delivered to the other user, but then allows the delivery to be un-done. However, in the later scenario, if the receiver reads the message before it is "unsent," then you aren't really providing a reliable undo facility.