[mjburgess]

This assumes an implausible view of rationality, knowledge and intent.

People who intend action-X do not also intend all the consequences of X, nor are even plausibly aware of what those are. And even when they are, those are often so numerous that a time-bound agent (a person) isnt going to enumerate and weigh each for most actions.

One common case is collective action problems, eg., the tragedy of the commons: by each fishing to their intended amount they harm their ability to fish in the future, which was not intended.

Social media is a very plausible case: by each posting to their intended amount, their create the unintended result of the "gamification & publication of social life as such", and thus harm their future ability to socialise as intended.