What do you think are the most important or biggest retcons in terms of how they diverted the flow of the story?

Are we including the Ur-Text? Because then I think AGOT counts as the biggest retcon. 

My biggest retcon peeves are the small things: Bloodraven being Maekar’s Hand, Aenys I not being married to his sister (although I may be mis-remembering things), Aegon V not fighting the War of Ninepenny Kings, etc.