MacBook Pro 15'' RAM Question I own a MacBook Pro 15 Late 2011 and I bought it for my work which involves crazy amounts of photoshoping every day. As PS is always hungry for RAM I bought 2 8Gb SODIMM modules to replace 4Gb modules that was factory installed.

I've tried to put 8Gb modules inside (which I've done before like 1*10^6 times before with other macs) and for some reason only one of them worked so I thought: "Ok maybe one of them is faulty I'll send it back", and I did. So after all I've upgraded my MBP15 from 8Gb to 12Gb of RAM.

Now (after one year LOL) I've suddenly realized that this 12Gigs is not enough for me and I want more. I started to search to buy a new 8Gb module to install it. Along with that, I've checked the google and found out that my MBP15 Late 2011 supports only 8Gb at max

http://support.apple.com/kb/HT1270?viewlocale=en_US
(context links don't work so just cmd+F and search for "15-inch, Late 2011")

So my questions are:
1. How it is possible for me to have 12Gb (8+4) right now? Is it safe?
2. Is it possible to boost it to 16Gb (8+8)?
3. Did anyone successfully boost MBP15 to 16Gb? If so please share the instructions.

I guess the other 8Gb I've sent back was not faulty and MB refused to work with it? Or I don't know ... I'm confused.
The 2011 Mac MacBook Pros have an Apple specified (as you found) maximum amount of RAM as 8GB.  The "real world" tests all show that the 2011 MacBook Pros actually have a maximum amount of 16GB of RAM.  See below for several examples.  This has been somewhat common practice for Apple over the years, where there specs will show a maximum of X but the real world tests and reviews all confirm a maximum of Y.

Your initial 8GB DIMM was probably faulty.  Had it been functioning properly your Mac should have recognized all 16GB of memory.  There are no technical reasons or firmware restrictions that would keep your Mac from recognizing the full 16GB.

Other World Computing's RAM upgrade page for this machine

Crucial's RAM upgrade page for this machine

EveryMac's spec page for this machine showing official &amp; unofficial RAM specs