I have unbricked 2 systems that got bricked and ran the exact same installer as what caused the brick on the same sd card etc and it worked fine the second time on both, so it's not just bad sd cards or incompatible sd cards or people being dumb and using some dodgy corrupt version, it's simply a glitch in either the program or possibly just in the way the 3ds handles reboots causing it to freeze in the middle of the process