I've really not been paying attention to this; I'm still working along on my busted-ass, cracked Galaxy 4 and I've got a ways to go before I'm due for a handset

I've really not been paying attention to this; I'm still working along on my busted-ass, cracked Galaxy 4 and I've got a ways to go before I'm due for a handset upgrade on my plan. Did Samsung really screw the design, or did they blunder into a bad batch of batteries? How many times has this catastrophic failure actually happened? Is it anywhere near frequent enough to make something like this a proportionate response to the risk?