3. The Entire Mandarin Debable - Iron Man 3
There's perhaps no single moment more divisive in the MCU than Iron Man 3's head-scratching attempt to reinvent the villainous character of The Mandarin.
Though the marketing built up the antagonist as played by Ben Kingsley, it's ultimately revealed mid-movie that Kingsley is really playing an actor named Trevor Slattery, a stooge hired to act as a smoke-screen for the "real" Mandarin, Aldrich Killian (Guy Pearce).
Unsurprisingly, fans of the character were left miffed that Shane Black had reworked him so drastically.
It's easy to appreciate the desire to move away from The Mandarin's outdated, stereotypical depiction in the comics, but replacing him with a boring white guy who hates Tony Stark because he snubbed him at a party once? That ain't it, chief.
Due to the backlash, this was effectively retconned in the MCU One-Shot short "All Hail the King," where it's revealed that there is another totally real Mandarin waiting in the wings, who will be played by the great Tony Leung in the upcoming Shang-Chi and the Legend of the Ten Rings.
Considering how effectively the movie's trailers presented The Mandarin as Tony Stark's most fierce foe yet, it was astoundingly lame to see him butchered so thoroughly on-screen, all in the name of being "subversive."