I believe there are three "rare" reactions that can occur - the one that gives you a 5-10 minute buff; the "flawless reaction/scribing, etc" which gives you an automatic pristine item; and then there is one that gives you a pristine item and I believe a rare item appropriate for your craft.
I have gotten the buff many times, the Flawless Reaction a couple of times, but have never ever gotten the 3rd one (which is why I don't even know what it is called).