Shortly after getting my 2009 Mac Pro in June '09, I went and installed a 3x4GB kit from OWC; No problems other than the Memory Slot Utility message after every startup. It never bothered me and the machine always reported the correct RAM amount; I should note these DIMMs have black heat spreaders on them. Faster forward to today; I ordered another 12GB(3x4GB) from OWC to bring the total RAM to 24GB; These DIMMs do not have the black heat spreaders, but I don't think that should be a problem. The machine cannot startup, and will sit there, with the power light indicator blinking continuously. I initially thought the sticks were bad, but I had the sneaking suspicion that there was some compatibility issues between the two sets; I pulled out the old 12GB kit and left the new one in, and lo and behold, it starts up. Has anyone encountered this with RAM? I know they're both kits from OWC, but I'm sure they're just rebranded DIMMs from whomever is their supplier at the time. I've already contacted OWC, but I'm hoping they'll replace the older 12GB kit if this is the case. Edit: I should also note, I've reset the PRAM, zapped the PRAM, and deduced that it was all the DIMMs and not just one bad one, by individually installing the older memory DIMMs with the newer ones and vice versa.