Shoggoth2588 said:
The ending of season 2 of Rosario + Vampire. A brief summary:
Harem following main male character: "Which one of us do you chose as your steady girlfriend?!"
Main Character: "I just can't chose a single one of you. I like you all too much to do that!"
Harem: "Hoo-raaay!"
This harem included a pair of mothers and, a girl who couldn't have been older than 12. This scene happens after the main character kicks the ass of the main female's father who it's heavily implied he'll end up with anyway.
To be fair, Rosario + Vampire was a harem show to begin with, the whole shounen-battle aspect only gained traction later, and I would hardly call a harem-series ending with a harem really a fuck-you to the audience. In fact, taken how deep the series explored the implications of the love-dodecahedron (or at least much deeper than any other harem show I have seen), the protag ditching all the girls for Moka alone would have been arguably an even bigger fuck-you to the fanbase.
Also, I have just realized that most of these "fuck-you" moments are related to endings, so I decided to bring up a non-ending example: Sasuke Uchiha's heel-face turn during the final stretch of the series.
You see, Sasuke has been repeatedly shown to be a complete monster, not above sacrificing his friends for the sake of power and revenge, all the while constantly whining (and I am not even saying that as some kind of hyperbole, just look up his actual dialog! I dare you to find a chapter prominently featuring him that doesn't have him moping about something).
Then, out of the blue, it is revealed that his revenge was misguided so that he immediately decides to stop trying to destroy his hometown and instead he want to become its leader, so he suddenly turns around and starts helping the heroes. And just to compound the bullshit, it turns out that his broken eye-powers are the reason for his acts, since they create eeeeevil chakra that in turn make him eeeeeeeeeevil himself, effectively absolving him from all the morally bankrupt bullshit he had done along the years, because you see kids, with wasn't his fault, it was the eeeeeeeeeeeeeeeeeeevil chakra all along!
I don't know what developed from all this, so what I just described might be out of date and he might have did a face-heel turn since then, but this bullshit author saving throw was the reason I dropped reading Naruto, so there's that.