Fandom Gank

The Fandom Gank is a Sister Trope to the Gainax Ending, and can easily be mistaken for it. Both can be confusing, or enraging, or just plain strange.

But a Gainax Ending is intentional -- it's designed to confuse or piss off the viewer, either for Grand Artistic Reasons or for Creator Lulz. A Fandom Gank, on the other hand, is unexpected -- the creator or creative team behind a property were attempting a Twist Ending or Esoteric Happy Ending that they thought would be cool and interesting, or appropriate to the setting/story, and expected the reader/user/viewer base to Get It and applaud them. However, instead of fascinating the audience and inciting discussion that enhances replay/rerun/etc. value, it ends up alienating a large portion of their fanbase, possibly to the point of being a Franchise Killer.

Expect a Fandom Gank to happen if phrases like "Avant Garde" or "High Concept" were being bandied about unironically in the writing department during development.

The important thing to remember when deciding a work is a Fandom Gank is that the story is almost always a good one, and might well be an excellent one -- until you get to the critical last few minutes or pages. A work that sucks or confuses all the way through is just bad. It's that sudden left turn into Epic Fail at the end of an otherwise enjoyable work which creates the Fandom Gank.

Compare, of course, Gainax Ending. See also What Do You Mean It Wasn't Made on Drugs?, which fans may ask the creators.

Anime and Manga
""Seriously? She could be the queen of a magical kingdom, and instead she goes back to high school, subway molestation, and a glass ceiling as an OL?""
 * The original ending of Neon Genesis Evangelion, before the movies, has been held up as a Fandom Gank by some viewers.
 * Some viewers had this kind of reaction to the ending of Vision of Escaflowne:

Video Games

 * The Trope Namer came out of the massive negative backlash to the end of Mass Effect 3 as it was originally released. The fandom reaction to the ending was equal parts "Huh?" and "You gotta be kidding!" and eventually grew to such a size that Bioware was forced to release DLC that tried to put a more acceptable spin on what happened.  It's debatable if they succeeded, but the franchise managed to stay alive to spawn 2017's Mass Effect: Andromeda, which seems to have redeemed the property.
 * The end of Fallout 3 spawned a similar response, but rather than a half-hearted effort like Bioware's, Bethesda's answer dramatically improved the endgame play.
 * The end goal of No Man's Sky, as promoted by developer Hello Games, is to reach the center of the galaxy. Players who went to that exact spot (this takes a lot of time) were shocked to discover that the end, contrary to their expectations, is simply the game zooming back all the way to its start, where another planet to explore is randomly selected, and all the tech they got from a lot of grinding breaks. What was probably meant as a "look at how much you traveled" kind of ending angered a lot of players.