Rivera is out of challenges for the rest of the game after challenging two plays in the first half. His second challenge was a questionable decision, even though he was right. While Rivera helped push the Broncos back seven yards with a correct challenge after a Peyton Manning sack, Rivera used up his final challenge of the game to do so. It's a decision he could come to regret if the officials make a mistake on a bigger play later in the game.
Rivera's first challenge came after Jerricho Cotchery bobbled a potential catch in the first quarter. While CBS' Mike Carey expected replays to overturn an incomplete catch, the replay officials stuck with the call on the field. It was a tough call to overturn. NFL coaches only get two challenges per game, unless they successfully challenge the first two. Then they get a third challenge.
In the heat of the moment during the second quarter, Rivera likely lost track of those rules and burned his final challenge. It was a lack of composure that he hopes doesn't cost his team in Super Bowl 50.