I had this problem too. Just got a black screen as I was using the phone..and turned out it was a reboot underway - it happend a few times on my 4S and has happend once in the week I have had the 5. Might be just me, might be iOS 6 issues... I would be interested to know if this is extremely rare or if other people have had similar issues. I can not repeat the error so thus have not escalated for repair, review etc.