A terrible giveaway by Navas led to a dangerous Barcelona counter-attack, which was capped by an ill-advised lunging tackle by Demichelis on Lionel Messi in the box. He got none of the ball and a lot of Messi. Referee Jonas Eriksson correctly pointed to the spot and sent off Demichelis for denial of a clear goal-scoring opportunity, and Messi converted from the spot.
Here's the foul, which was a terrible one. Demichelis is well-noted for his regular terrible errors in judgment, but even for him, this is an all-time great gaffe.
While Demichelis had no business going in on Messi like that, Navas is just as much at fault for the goal. He had two Barcelona defenders in front of him, none behind him and plenty of teammates in support when he gave the ball away. He could have made a sideways pass that helped to advance the attack, but instead attempted to go alone and set up Barcelona for an easy chance.