What happened is fairly simple. In Warcraft III, we're told that the eredar were a race of sorcerers and warlocks whose corrupt magics date back to the dawn of time, devourers and corrupters who ran afoul of Sargeras before the titan went mad. In fact, in the original story, it was his encounter with the eredar that started Sargeras down the road that would lead him to go mad. At this time, no mention was made of the eredar being in any way related to the draenei, nor were the draenei depicted as anything beyond the deformed model used by Akama. Even when World of Warcraft debuted, the few draenei in game were known as lost ones, such as Magtoor or Kum'isha the Collector, and they were nothing like the draenei we have today.
The obvious lore contradiction with Sargeras and his encounter with the eredar was clearly documented in the Warcraft III manual. I wrote those bits about four years ago, and to be totally honest, I simply forgot.
-- Chris Metzen, Metzen on Lore
And then the mistake happened. Mr. Metzen, in preparation for The Burning Crusade, came up with a way to link the draenei of Outland to the eredar and in so doing, completely contradicted what he himself had written in the Warcraft III manual. And in so doing, he made the game as a whole much stronger.
It's no secret that I'm a big fan of the draenei as they are today. One of the things I like about them is their contradictions - how they chose to refuse Sargeras and avoid becoming the man'ari eredar, how they've spent generations fleeing from the Legion led by their own former kin, and yet how in their hour of darkness on Draenor they showed that they're not perfect -- there is the capacity for fear and persecution in the draenei heart, even though they've been so persecuted themselves. For when Chris Metzen created the new origin of the draenei, he created a people who had more in their character than the shadowly skulkers of Warcraft III who under Akama joined with Illidan to defeat Magtheridon.
In creating this new story, what Chris Metzen did was to show that the eredar were not all of one mind back in the distant past -- the modern draenei were those who chose to follow Velen, and refuse Sargeras (and their children and grandchildren), and furthermore, even after tens of thousands of years of exile these draenei still weren't a hive mind. The presence of the naaru hadn't made them all fall lockstep in line with Velen's vision. When some of their own people were deformed and mutated by fel energies used by orc warlocks, the noble and Light worshipping draenei turned on them. Out of fear and disgust, they shunned their own kind.
This stain on their selves is one that highlights them, it creates a distinct and unique kind of story, one of many that the draenei present. The draenei history is one of a people at war with themselves and their own dark reflections, seeking to come to terms with their own potential for evil, both within and without. How the lost ones and broken were treated shows that the draenei are far from having mastered that conflict. Their fear of corruption led them to behave in a way one might wish to blame on corruption, but it was no demon and no warlock who led the draenei to betray their own. The broken and lost ones thus serve as a perfect objective correlative revealing the forever imperfect, divided heart of the draenei people - who themselves are a lost portion of the soul of the eredar, a time capsule of the way life on Argus would have been, twenty five thousand years ago before the coming of Sargeras.
And we wouldn't have had any of this if not for that one mistake. I'm very glad it happened. We wouldn't have gotten the excellent story Unbroken without it. We wouldn't have had The Burning Crusade or Velen or years of jokes about the Exodar crashing without it. We wouldn't have the draenei I've grown to love playing and writing about - who contain all that we had before the mistake, and a whole lot more to boot. If Chris Metzen had gone back and read his old notes for Warcraft III, we might not have had a lore mistake that made the forums irate for a few months, but we definitely wouldn't have had what we got. I'm glad he didn't.