The video is above, and you can see that Carnage wins the fight by leveling Spong with a vicious right hand. Although Spong gets up before the ref reaches the count of 10, the ref looks into Spong's eyes, sees that he's wobbly, and waves off the fight, declaring Carnage the winner by technical knockout.
But just as the ref waves his arms, Carnage lunges forward and throws another combination, knocking Spong to the ground again, after the ref had already declared the fight over. That's where the controversy comes in.
Some observers (including play-by-play man Michael Schiavello, who you can hear in the above video) thought it was a dirty move by Carnage. Others (including color commentator Mark "The Hammer" Castagnini, who had a heated argument with Schiavello) thought the ref did a bad job of signaling that he had waved off the fight, and that Carnage didn't realize the fight was over.
And the controversy didn't even end with all that. After things had begun to settle down, the ring announcer (Cedric The Entertainer, believe it or not) took the microphone and announced that the fight had been declared a no contest because Carnage punched Spong while the ref was waving it off. You can watch a longer version of the video, including Cedric The Entertainer trying to provoke the crowd into demanding a rematch, here.
But while Cedric The Entertainer was excited about the possibility of a rematch, the result makes no sense: Once the ref waved his arms, the fight was over, and Carnage was the winner. If the promoters want to punish Carnage in some way, like withholding a portion of his purse, that's fine. But to simply say that the fight is a no contest is ridiculous. Spong lost the fight as soon as the ref waved his arms, even if Carnage didn't think the fight was over.