Quote Originally Posted by tdb View Post
I seem to recall Dumbledore warning them about catastrophic consequences if they messed up. But since everything went according to plan, we never found out what those consequences would be. Maybe you'd end up a ghost or something.
I've still got the book so I checked. No specified consequences, just "you know what is at stake - you must not be seen". (With the implication that Hermione had some kind of longer explanation of the rules in the past, but we don't get it.)

Of course it's more like "you must not be seen and recognised as being where you shouldn't be" because simply being seen - as it happens in the end - doesn't cause anything to happen at all. It's more about not creating a paradox by letting their past selves see something they didn't see the first time.

There are two different takes on that. One is that it's simply a self-fulfilling prophecy sort of thing and as long as you try to hide and go unseen, you'll be successful because these events already happened and you weren't seen. If you deliberately tried to break things, the universe might conspire against you and send some unexpected event to prevent you from successfully contacting your past self.

More likely, you simply do have to be careful and if you do create a paradox, the consequences could range from "we don't know but we don't want to take the risk" to "total collapse of the space-time continuum" or, as in FFXIV (or at least my take on it), causing a second timeline to split off the original and take you with it, never to return to the point you departed from. To an observer in the first timeline you would use your time-turner and that would be the last time they ever see you.