hi, guys!
I know this is too late to reply, but this for all the people who will come to this page after typing in google smth like "error 12; iphone; omg please help me..."
Sooo, this is my story:
one year ago my iphone 5 was swimming in the toilet,i put it in rice, everything was working despite of back camera, but it was okay, and i didnt plan to fix it. I used iphone 5 for one more month and after that i got iphone 6. In like 2 month i connected my iphone 5 to itunes and i wanted to install the new IOS. And this **** happened... error 12, screen was white with apple logo, after it turned grey and then it turned off. I tried to google my problem, but nothing came out, so i forgot about this problem and just put my iphone 5 back in package.
After one year, so - today i remembered that i have this iphone and i tried to fix the problem again.
i put iphone in recovery mode few times, it didnt work, then i saw a video for another way, i downloaded ReiBoot, and tried there, it didnt work again, then i tried again in itunes - didnt work. Then i thought "okay, i try the last time, if it will not work - i give up".
I TRIED AND IT WORKED! I didnt do anything special... Every step that i did before i just repeated.
This is really weird, cause i actually thought that i 'lost' my phone one year ago, but now some magic happened.
I hope my story will help somebody. So if you have a lot of patience - just try again and again.