Undertale is an indie game that promotes and encourages kindness toward others. You can play the game however you want, and there are a multitude of endings depending on how nice/mean you are in your playthrough.
But if you’re not 100% kind to everyone you meet; if you take even one unkind action toward someone, you’re locked out of the perfect good ending. And it remembers your playthrough, so you can’t ever earn it by replaying the game. I dunno if that’s been patched; I haven’t played it since about 2015, but that was the rule when I started it.
And there was no indication starting out that you had this choice. Most people default to fighting bad guys in games. There wasn’t even a hint that you could play the game as a passive, kind person and never harm anyone, despite their aggressive and harmful actions toward you.
So most gamers got locked out of that perfect good ending. Which I guess is kind of the message of the game. Every small act, whether good or bad, can affect people around you permanently. But it’s still annoying as a completionist, knowing that I could never perfectly complete a game because of a rule I wasn’t informed of when I started.