Early in my experience with the Kindle DX, which I love and use constantly, I put the default Amazon case or cover on it. The cover attaches to the Kindle through two metal tabs that engage in the side of the Kindle’s plastic case. It’s not a bad cover, but it turns out that if you open the cover upside down accidentally (easy to do since the nondescript black leatherette looks about the same apart from the Amazon logo), the metal tabs flex the Kindle’s case and it can become cracked. Mine was within 2 weeks of getting the device, but without any real damage. I kept using the Kindle since I didn’t want to hassle with returns, migrating content, or being without my Kindle.
Last Friday night, I open an email from Amazon, and it contains a friendly reminder about my Kindle warranty and what it provides me. And in the middle, a little paragraph precisely describing what can happen if you open the default cover/case backwards — describing the cracking I’ve got. And the email encourages you to get in touch with Support.