The cheap and easy answer to this one is "Every Lost season finale." And it's an accurate one, but... it's cheap and easy. And there's a better answer.
Picture it: It's late April of 2009. It is, to be more exact, my birthday. And tonight is the second season finale of Chuck, the show that never fails to make my heart happy. But my TV intuition is acting up, and it's telling me this is going to be a bumpy ride. A major character death has been promised by the teaser leaks, and even though Bryce Larkin is scheduled to drop by and I've seen television before some part of my brain is convinced that it still might be Casey. And the show has yet to be renewed for a third season.
The hour proceeds at a manic pace. The plot is wonderfully mad, inasmuch as I can concentrate on it when I'm this worried. Casey survives, thanks in no small part to my tireless shouting at the television. Bryce dies. Chuck makes the conscious decision to take on the New Intersect. They get attacked. And then... Oh, and then. Our sweet, gawky little nerd takes a level in badass. One massive kung fu explosion later every one of his opponents is on the floor, he's bewildered, Casey and Sarah look mildly terrified, and...the season ends on the biggest conceptual cliffhanger ever.
And it's weeks before we find out that we're getting Season 3.