on some devices when you root or modify the core system it will trip a digital flag, more or less voiding the warranty and it cannot be removed, samsung in particular added a nasty system
http://androidforums.com/threads/guide-q-a-myths-and-truths-about-knox.807185/
http://androidforums.com/threads/guide-q-a-myths-and-truths-about-knox.807185/