The FIA has told four teams (McLaren, Mercedes, Renault and Force India) that they must modify the diffusers on the back of their cars to comply to the regulations set forth for all teams. The FIA deemed that these teams had exploited a loophole in the rules that allowed them to fit larger starter motors, thus having larger openings in the diffusers for those starters. McLaren were also the subject of controversy earlier in the season when they came under fire for having a scoop on the front of the car that channeled air to the rear wing, improving aero efficiency. They were cleared of that charge, but this one is going to stick and they, along with the other three teams, will have to have the modifications in place by this weekend, when the Australian GP takes place. Here is a link to a little more info on this.