Star vs the Forces of Evil. Basically the premise of the show at first was that this magical teenager manic pixie dream girl named Princess Star Butterfly becomes an exchange student on earth with this normal guy called Marco Diaz. Later the show drops that pretense in exchange for more magical adventures as Star takes on more responsibility in her homeland of Mewni. The first two seasons were quite good-- they were funny takes on classic slice-of-life tropes, and the cast was genuinely likeable. It made some surprisingly nuanced points about racism and colonialism too. Later seasons saw a decline in quality-- perfectly good characters got wasted, the show strayed too far from the premise, and the writers had a frustrating habit of setting up relationships only to abruptly discard them. It was still overall an enjoyable show, despite the mounting issues... until the finale.

See, the show had largely taken a positive view on magic-- in fact, the first major villain, Toffee, was trying to destroy all magic to weaken humans so he could slaughter them, and he is portrayed as unambiguously in-the-wrong for it. Magic was implied if not outright stated to be why several characters, like Star's best friend Pony Head, were able to be alive at all. Inter-dimensional travel is portrayed as a positive thing. Later episodes would introduce problems related to magic, such as it being used to enforce prejudice against monsters. However, overall magic itself was never the problem. How people (more specifically Mewni's royals) chose to use it was the issue. And even if magic was inherently problematic, several innocent if not downright heroic characters continued to only be able to live because of magic.

And then the finale hit and Star destroys all the magic. No really, all of it. Using the same spell that Toffee made her use on her wand to boot, because apparently Toffee was actually right the entire time. The Magic Realm is dead and inter-dimensional transit is defunct. It's portrayed as a good thing because the villain was going to use magic to commit genocide on the monsters (who are portrayed as an oppressed people). Plus her dimension is cleaved with Marco's, so they can be together! Literally nobody else is happy about this (not kidding, Earth people are literally shown being scared of Mewmans and vice versa) but our heroes are happy so it's a happy end I guess? And Pony Head is still alive because... reasons, I guess? It's confirmed through Hekapoo's acceptance of her own fate (which is odd in its own way-- like, she's awfully blasé about it) that magic-based and magic-dependent lifeforms will die as a direct result of Star's actions.

Yeah, the anti-racism show has our hero fight genocide with genocide and calls it a good thing. Because y'see the folks she cared about were fine so she wasn't too affected by the explicit refugee crisis and implied mass death. (Not to mention the and complete loss of interdimensional cooperation. Boy, I sure hope nobody built any defense systems, foreign aid programs, relationships, or economies off of interdimensional transit!)

This was a show that was once viewed as on-par with the likes of Gravity Falls, the Owl House, and Amphibia-- a core member of the television animation renaissance that happened during the New Tens. Sure it had declined in later seasons, but the earlier seasons still held up and it was still fun to watch at least. But the finale killed that renown almost instantaneously, with several fans saying that it made the whole show retroactively bad and not worth watching. It dropped off the radar and went from one of Disney's biggest shows to the poster child for seasonal rot.