The tracking on the lasers for CDs is pretty crazy, since at those scales even well balanced CDs wobble like crazy. If it had to be super flat for it to work, each disc would be much too expensive. And as soon as it got dirty or warped in the sun, it wouldn't work anymore. In reality CDs are pretty rugged and can take a lot of abusive before they can't be easily read in even a cheap reader. It's amazing technology really. It's kinda crazy to think about how many holes per sec the laser can track and read for something like a blu-ray disc running at multiple times playback speed for data transfer.