Though generally pretty sturdy, Apple's MacBook line suffers from a slight design flaw that results in the front-facing edge of the case developing cracks and eventually losing small splinters of black or white plastic. This happens because of the pressure that users' wrists exert on the top of the case, below the keyboard. The outer edge of the top case plate, about 2mm, is much thinner than the surrounding material and is prone to cracking:
This is basically an æsthetic, rather than structural, problem. Nevertheless, there's an entire Flickr Group dedicated to the problem. The consensus seems to be that Apple will replace this part of the case if your laptop is under warranty (or AppleCare.) But for those of us whose computers are beyond the 1-year time limit, there's another option: replace the top case plate yourself.