See http://forum.xda-developers.com/showthread.php?t=2447832 Basically, newer firmwares for Galaxy devices etc are going to have Samsung's new KNOX security system in place on bootloader level. It can detect if you have rooted the phone or otherwise tampered with it. Because of this it shows a flag in download mode and this can cause you to void your warranty. Not because the KNOX flag actually breaks anything, just because repair shops have been instructed to deny warranty on any 0x1 flags. So far the KNOX flag can't be unset like the flash counter can. Hopefully someone figures out a way eventually. I'm still on the MGA firmware on my Galaxy S4 so I can do whatever I want still, but if I were to update to a newer version of TouchWiz, then the bootloader would get updated and even downgrading to an older stock firmware would set the KNOX flag.