3. Lost - Season 3 Finale (Through The Looking Glass)
The season three finale left everyone scratching their heads as the 'flashbacks' shone a new light on our heroes, suggesting that perhaps Jack and Kate had met each other before the Island and that there were greater mysteries audiences were just beginning to uncover. It wasn't until that final scene that the rug was properly pulled out from under the audience. Viewers were already left devastated by the death of Charlie on the island. But they still didn't realise until the very end that they were't looking at the past; they were looking at the future. Jack and Kate hadn't met before the island. Audiences were looking at a flashforward. Suddenly Lost was sent in a startling new direction, rejuvenating it after a slow, repetitive third season and leaving everyone desperate for season four.