12 Biggest Retcons In The Marvel Cinematic Universe
5. S.H.I.E.L.D. Has Been Infiltrated By Hydra All Along
Hydra's been the subject to several retcons across the MCU, which is only to be expected when you have an organisation built almost entirely on "shocking" twists (at least cinematically).
The biggest is the series' most competent retcon, a massive reveal that all along Hydra had been a part of S.H.I.E.L.D., influencing the organisation and major world events for their own chaotic gains. The scale of the twist goes beyond just Robert Redford being the movie's inevitable "famous actor villain" though; you have several characters - Agent Sitwell, Garry Shandling's Senator (yeah, that was a thing) - realigned as undercover agents, which in turn influences their early adventures - Sitwell in Agents Of S.H.I.E.L.D., Shandling's desperation to get Iron Man's suit - and the entire existence of Cap in the modern day.
Retcon is often used as a dirty word, a quick put-down to warn creators off tinkering with accepted facts in their work, but the deftness of this twist (which wasn't decided on until the movie's development) shows that it sometimes can be executed in a very respectful, forward-thinking fashion.
If only the movies that had followed had dealt with it in a similar way...