I will never forgive that one BC retcon. Draenei. Ugh. Initially it was written in the WC3 manual that Sargeras was a good Titan defending worlds from demonic invaders and locking demonic races up. The big ones being Eredar and Nathrezim. Sargeras defeated them but couldn’t understand their motivations and eventually fell to their corrupting influence, released them and all the other demonic races he had defeated, and formed the Burning Legion.
Then in TBC, Sargeras was actually a big bad all along, and Eredar were a good technologically advanced race that were then corrupted by Sargeras, with Velen and a bunch of others fleeing to a planet called Draenor and named themselves Draenei, which in their language coincidentally meant the exiled. Oh and those ugly Broken were retconned into being mutated eredar, instead of being the natives to draenor that they initially were.
That’s why people were so confused about the leaks of their new race. It was already well known Horde were getting a pretty race with the Blood Elves, but when a leak said Alliance gets Draenei everyone assumed they were getting the ugly Broken, for their token ugly race. Then there was an image leaked of the Alliance race and people were shocked to see playable Eredar. Then it turned out that the Eredar ARE the Draenei…
And what was the given reason for the retcon? Chris Metzen forgot.
The only other retcons I hate so much is all the Twisting Nether and Demon retcons in WoD and Legion… ESPECIALLY because it led to Illidan coming back and the DH class being a thing. Disgusting.