I'm doing my own LOST rewatch with a friend of mine who has never seen the show, so while he's watching things for the first time I get a refresher headed into season six. Tonight, we're watching the Season 2 finale and Locke says to Desmond that they needed to not push the button in the Swan station, and that he's "never been more sure about anything in his life." I thought it was amazing how much that mirrored Jack's rhetoric when it came to detonating the Jughead at the Swan site at the end of Season 5. Like Locke was wrong at the Swan in Season 2, I think it was a clue that Jack was wrong at the end of Season 5. I think this confirms the theory of "Whatever happened happened" and that Jack is unable to change the future, only to cause it.