My initial comment about 'Any time you mix set...can be problematic) is simply true and happens all to often, many people who have mixed sets with success go back to DDR, DDR2 and early DDR3 sets that were at 1066/1333, these days 1600 is sort of a toss up and the higher you go the more problematic things become, I'm an Admin ofn Gskill's forum and deal with these issue every day, here though many will tell you there's no problem as long as the timings, voltage and freq are the same, they don't take into account, the multitudes of ICs (memory chips if you will) that are used these days, or the different advanced timings, etc. Packaged set are tested so that all the sticks work together, if it was as easy as many want you to believe, DRAM manufacturesr would simply sell nothing but single sticks, charge more and make more - but they can't do that, if they did their RMA rates would soar, as it is, with GSkill about 80% of the sticks that are RMAed are perfectly fine, not a single thing wrong with them....the reason they didn't work is someone wanted 4 sticks and saw 2 2stick packages was $10 less than a 4 stick package and they ended up with sets that didn't want to play together