Even solid brake rotors crack. That's how hot things get under racing conditions. This is a stock C5 brake rotor. The small cracks came from normal racing, but the rotor finally decided it had had enough. You'll probably hear a crack this big as you apply the brakes. This is why you need to inspect your brake rotors between every session. This applies to those of you who run track days, too. You're braking just as hard as a racer brakes.