When CM Punk got a shot against ROH Champion Austin Aries, it was well known he was leaving for WWE, and most fans expected that show to be his last. Either he'd put over Aries on his way out or he'd get the title and surrender it to set up a tournament.
Nobody expected what actually happened.
He won the title and cut the expected emotional promo before weirdly getting sidetracked into sharing a parable with the fans:
"There was once an old man walking home from work, and he's walking in the snow and he stumbled upon a snake frozen in the ice. He took that snake and he brought it home, and he took care of it. And he thawed it out, and he nursed it back to health. And as soon as that snake was well enough, it BIT that old man. And as that old man laid there dying, he asked the snake, 'Why? I took care of you. I loved you. I saved your life.' And that snake looked that man right in the eye and said, 'You stupid old man. I'm a snake.'"
To sum it up, Punk's entire time as a babyface had been a ruse. He was building up fan support to help him get a shot at Aries, and once he had the belt, he dropped the facade. Not only that, but he was taking the belt with him to WWE.
Christopher Daniels (in a surprise return after an absence when TNA wasn't letting ROH book their talent) then showed up to stand up for ROH, but Punk ran away with the belt. Punk continued to defend the title for a couple more months, cutting awesome promos throughout (including one where he signed his WWE contract on the belt, which unfortunately isn't online) and eventually lost the belt to James Gibson (Jamie Noble in WWE).
Gibson was soon re-signed by WWE, but promised to do the opposite of what Punk wanted to do: As long as he held the title, he wasn't leaving. About a month later, he dropped the belt to Bryan "Daniel Bryan" Danielson.
If you want to see the storyline play out, it is featured in the "Summer of Punk" DVD set.