When the protagonist gets so pissed of at a villain that she kills her, on purpose, and doesn't feel bad about it afterwards.
When a feminine looking and acting character of undefined gender actually turns out to be a boy, not a girl badly pretending to be boy, and not because he is in hiding, but rather because he just is that way and likes to dress like that, and then falls in love with another boy, and they have a happy gay ever after.
When the protagonist at the end of the story decides that the villain was right all along, and changes sides.
When the world actually ends destroyed despite everything, including everyone.
When the hero decides to not sacrifice himself, and tells others they have to deal with their own problems, because fuck it, and then goes on his way, and doesn't get shamed for it by the author.
When a boy who is constantly punched, slapped, and degraded by his bitchy female abuser, punches her right back, and is applauded for it, and she is called out on being abusive and horrible, and the boy finds better friends.
When the chaotic anti-hero isn't given a tragic ending and doesn't die, instead everything ends up perfectly for him.
When the friend/little sister/little brother/child of the protagonist who was stolen for cheap angst to make the protagonist watch as he is killed, and to make the protagonist angst, actually doesn't die and is rescued, and the protagonist doesn't angst but instead acts in an extremely efficient way and wins.
When the villain uses the loved one of the protagonist as a shield, and the protagonist completely disregards the threat and goes for the kill, and wins.
When in the end the main character chooses as his romantic partner his childchood friend, rather than the girl next door who he just meet a week ago.
In case these weren't sufficiently mystery-centered, here's something more mystery-centered:
When it turns out that there actually is some kind of magic/alien/monster behind the great mystery.
When in the end of the story we find out that everything that happened is, in fact not an allegory/metaphor about the internal psychological struggles of the character, neither is it her delusions, but all the weird, mysterious, monstrous, magical things really did happened, monsters are real, and magic is real, and there are other realities.
Mostly it's a list of plot twists which I would love to see, but which usually (and some almost always) go the opposite way round to my big dissatisfaction.
And I kept it non-specific so there's no titles or character names for you, only plot twists and plot devices.