One mistake. It was just one mistake. ONe small tiny little sliver of overlooked information that would cause the downfall of the wizarding world. The one responsibe for was to be latter known as the biggest fuck-up in all of wizarding history, was none other then Albus-to-many-names-Dumbledore. It wasn't that the mistake itself was so terible, some would even say that it was a good thing in fact. It was just a simple sentence in a single book that lay forgotton in hogwarts library. It may have been mentioned in passing to a few close friends, or simply forgotted in the rush of new knowlege as soon as it was read.
No it wasn't the mistake itself that was so bad, but rather the consiquences of the mistake that destroid any and all chance that the light side once had at wining the war. The mistake that Dumbledore had made, was failing to fully research all of the details of a horcrux. Oh sure he had all of the 'big' stuff such as what they where and how to destroy one. In fact he knew almost everything there was to know about horcrux's. That was, everything except what all who had ever created a horcrux agreed was the singular most important thing about a horcrux. The single forgoten scentence simply stated
'ALL HORCRUXES ARE SENTIENT, THIS ALLOWS ALL HORCRUXES TO COMUNICATE WITH THE MAIN PART OF THIER SOUL, AND IN CASES OF MULTIPLE HORCRUXES WITH EACH OTHER'