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.