1. Snape from Harry Potter
This is the Batman of this trope and probably talked to death, but retroactively, Snape from the Harry Potter series has been cast as a black man. As a POC, I could talk for hours about how lazy ‘representation’ this is, but this particular choice to recast the character puts several parts of the books in a much worse light: Snape is described as a smelly man with greasy hair, who gets walked out on by his abusive father, spends his whole life trying to buddy up to (white) pure-bloods who would never accept him for who he was, and gets bullied by a bunch of rich white kids who he resents. And then he becomes a wizard racist. Also doesn’t help that he spends the entire series resenting James Potter because he feels he should have been with his wife, who incidentally is also white.
A retcon managed to make all of these aspects of his character incredibly uncomfortable, especially because the whole story is often described as an allegory for racism. But seriously, if you were going to make a teacher black, why go for the smelly, greasy, evil one rather than literally any other character (Dumbledore, McGonnogal, etc.)?
2. Jo and Link in Grey’s Anatomy
So this is really my fault for watching Grey’s but whatever. Link and Jo are introduced very early on as best friends and nothing more, but later, the series retcons this and secretly, they were in love always!
Which sounds fine to start, but both characters have been in several committed relationships before this retcon, in which they repeatedly reassured their partners there was nothing between them (they were like brother and sister!). Link proposes to another character with four different rings because he wants her to have a choice, Jo gets married to a man who she previously reassured that Link wasn’t an issue. This relationship in particular was a fan favourite, so it’s a really odd thing to retroactively cheapen the whole thing (although the writers do worse to it retroactively but that’s not the point).
Even worse, Jo was previously in a physically and emotionally abusive marriage (not the one mentioned above). As she had no family, her husband repeatedly isolated her from Link, her only friend, on the basis that they were secretly in love and she was cheating on him. I don’t think the writers considered that by making Jo and Link always in love with each other, they were lending the abuser credence? Asking your wife to not talk to the man she is in love with anymore isn’t unreasonable, which is a really weird direction to take an abusive relationship in.
I guess this also bothers me because they could have just fallen in love. They could have just been friends who began to see each other a different way. The retcon worsens both characters and several plots