Originally Posted by
Mr Mystery
So GK Terminators, and indeed the entire Chapter being equipped with it was the retcon.
As were all Aspect Warriors. Back in the days of Codex Titanicus, which as far as I'm aware introduced the first proper Eldar background (saving their souls up so they could all translate into the Warp as a single entity, birthing a new God and incorporating Slaanesh)? No Aspect Warriors. At all. At all at all. So best object to all Aspects, especially those Johnny Comelately Warp Spiders and Phoenix Lords (first showed up in Codex Eldar, and certainly for the Warp Spiders, possibly the Phoenix Lords, a WD article)
What else?
Space Marines being genetically modified to the extent they are no longer really human? Retcon.
Most of the Horus Heresy, and specifically Horus being a Primarch (and thus Primarchs themselves)? Retcon.