My MBPr just came back from warranty service where Apple replaced the motherboard. Previously, they had replaced display. I was having funky (black boxes) display issues.
I had purchased an 11" MBA to take with me on a two week motorcycle trip to Alaska, so I did nothing with the MBPr until we returned. It seemed sluggish. Then it was clearly sluggish. And I've had a couple of those black boxes block Safari. And it ran out of RAM (16GB) one day this week, while I was at work with the MBA. It has NEVER run out of memory, and I've had it since April 2013.
Now it may require a hard reboot after going to sleep. It has also reboot itself, spontaneously, while I was working.
The bottom rear dead-center is hot. So I ran the short test and got the same error that brings people to this thread: 4HDD/11/40000000: SATA (0,0) single pass.
I'm backing up the machine again, even though it probably has nothing new on it. (Most of my work lives on DropBox.) I'll run the 1-hour test and see what it says. Then try Genius Bar.
I'm @kegill on Twitter.