1. School Days: Nice Boat
At the start of the series, School Days presents itself as a -- perhaps somewhat dour -- slice of life romance about three high school students. Which is to say it looks a lot like the hundreds of millions of other slice of life romances familiar to the genre.
There are signs about what's to come. Namely, that the central trio -- Makoto, Sekai, and Totohona -- seem about as likeable and clued in as toilet bowl cleaner. Makoto is our male lead; one who's infatuated with Totonoha. Sekai is his homeroom classmate. One day she sees that Makoto has enshrined Totonoha -- a woman he's never spoken to -- as his cell phone wallpaper.
Rather than find this creepy, off-putting, or to be an immediate red flag Sekai offers to help Makoto hook up with the third party. While this seems out-of-nowhere at first, it quickly makes sense as the viewer discovers just about every major player on this show is a half-empty bag of dirt. Or at least as nice as one.
Makoto takes the cake, using the two girls' affection for him against them while he bangs his way through freshman year. Despite his constant, non-negotiated stabs at polyamory the rest of the cast is consistently shocked by Makoto's seesawing unfaithfulness. Sekai eventually tires of this, yet for some reason remains infatuated with Makoto.
Rather than dump him like a logical person, Sekai follows the series ridiculous script and comes to the assumption that she's pregnant with Makoto's child. A presumed fact she announces to the entire student body, finally cutting off her would-be beau from his revolving door of sexual partners. In perhaps School Days' only realistic interpersonal exchange, the two fight. Makoto goes back to Totonoha, who has been so emotionally traumatized by this point that it seems like a good idea.
At which point things go from bad, to brutal. Sekai gives Makoto a good, old-fashioned stabbing.
After which she's understandably surprised to get a message from the dead dirt-bag's phone. It's Totonoha, who reacted to Makoto's murder by lopping his head off and dragging it around with her. She completes the set by slitting Sekai's throat...
...and then confirming the pregnancy was a sham in the most violent way possible.
After which the many times jilted lover literally sails into the sunset with her one-time boyfriend's noggin. End scene, cue the romantic music.
2. Neon Genesis Evangelion: Congratulations!
Most of you reading probably saw this coming. Neon Genesis Evangelion is such an obvious "WTF ending" that perhaps it's not even worth mentioning. It is, after all, the complete bullshit finale that defined complete bullshit finales. That said, we'd feel remiss if we didn't at least mention the ending so completely cuckoo that the world had to invent a new phrase to describe it.
If you aren't familiar with Neon Genesis Evangelion (to be referred to as Eva from now on to preserve our own sanity) the series is the classic deconstruction of the mecha genre. Mobile Suit Gundam popularized the idea of teenagers fighting each other in giant robots some 15-odd years earlier. Eva took it a step further by asking the question on nobody's mind. "What if that was actually the worst?"
Enter Shinji Ikari. A protagonist who is, appropriately, also the worst. His father Gendo, also-also the worst, conscripts Shinji to fight "angels" from the cockpit of a temperamental bio-mechanical mech. What follows is a dwarf star's worth of teenage angst, misunderstandings about how sex works, and religious overtones.
None of that really matters by the end of the series, because everyone dies. Literally everyone. In a shocking turn of events the organization using child soldiers to fight religious symbols turns bad, and successfully bids to "unite" humanity into a single soul... by turning it into ooze.
Rather than face what's happening, Shinji realizes he can determine his own reality around himself. Which manifests in the final episode of the original series as a light-hearted high school comedy. During which Shinji experiences the now-internet-famous montage of his mecha pilot partners, relatives, friends, enemies, and a penguin congratulating him for realizing that he needs other people in his life. Or something.
The conclusion was so bizarre -- even to many long-time fans/apologists of the series -- that a two-part film finale was released more than a year later. The intent was presumably to clear up or provide an alternative to the bonkers resolution. The plan backfired since this ending -- which sports Shinji masturbating to, and later strangling, his unconscious friend -- was nearly as weird as the first. And so as with most things the conclusion to Eva is best left up to personal interpretation.