I have seen the kind of thing above (e.g picture from Stretch) on some GM rear brakes, the metal actually rusts somehow underneath the wear surface and becomes impressively rusty and eats pad.

But I have had brakes that looked like came from underneath the sea for 20 years on some other cars, and just started driving and looked new after 15 miles or so. I dunno what is the difference between them.

I think "shape of pad" is usually pad material that has stuck to the rotor. My CLK has this issue, the rear rotors have pad material stuck on them and when you brake, the pedal vibrates. I am not sure what to do, maybe will take a sanding disc to the rotor... new rotor is cheap but I am lazy and brakes are decidedly unsexy.