iMac 27" late 2012 unexpected shut down multiple times in a day:
Apple store Fountain Gate replaced the Hard drive on my first visit 28th Nov 2015. The problem still persist. On 10th Dec 2015, my 2nd visit to Apple store Genius bar they replaced the logic board & power supply. This is few months before my Apple Care expires which is Feb 2016. I'm all smile knowing the problem will go away BUT IT DIDN'T. My most dreaded fear. The random shut down didn't happen after the 3rd month post repair, March 2016. From December to February I hardly use my computer. I only use my iMac mainly for browsing & youtube that time. No heavy app. My guess is the 27" iMac late 2012 is generally a bad apple.
So replacing the logic board & power supply didn't fixed the annoying problem on my iMac. It means I paid for Apple Care for NOTHING. I am now out of warranty with this lemon. The title of "Proud Owner of the most advanced software/hardware, most elegant design desktop computer" was cut short by the iUSDS aka "iMac unexpected shut down syndrome"
(and they also changed the display panel, LCD because they accidentally brake it...and that delayed the return of my iMac for another week). The genius lady wasn't very happy when i picked up the iMac from repair. Did she broke it? I don't know. GENUIS!