An Accusation Fic is a Fix Fic or Revenge Fic with a Fandom-Specific Plot of type B -- that is, with a plot unique to the fandom it's written in. It is generally written directly because of Canon events.
Perhaps the actions of the lead characters demonstrate Moral Dissonance. Perhaps there was a What the Hell, Hero? moment in which the canon author sided with the hero but the Fanfic writer didn't. Perhaps it's Values Dissonance between the author of the canon and the fanfic writers.
It doesn't matter where the original wrong lies—whether it's a misguided author or a Misaimed Fandom. The result will be a plot in which the canon events will be deconstructed, or the results of the events turns dark. A careless author will use Diabolus Ex Machina for this; a clever one, using suitably flawed material, will make the consequences seem natural, or plausible if we've hit an Alternate Universe Fic, which it often will if the canon is long-running and the fic is about an event set midway through the run.
There will be negative consequences for the characters who did the perceived wrong. There may or may not be negative consequences for innocent characters, depending on how cruel the author is feeling.
Naturally, this sort of fic will tend to be a Dark Fic.
- In Lilo & Stitch: The Series, the eponymous girl and her illegal genetically engineered alien are tasked with capturing the other 625 experiments and repurposing them before the bad guys get to them. However, some would naturally be caught in order to avert a Boring Invincible Hero. Some fans cried foul about how Lilo appeared to seem to just let some of the captured experiments go. Experiment 166: Fury (which may in fact be the first Accusation Fic, depending on how old the genre actually is) invents a specially-made fan experiment that calls Lilo out on this, and at the end the she and the others go and rescue the until-then captured experiments.
- From House fandom:
- At the end of season 4, Wilson asked House to risk his life to save Amber's. House did. Then Wilson broke off his friendship with House because he failed to save Amber, even though House did what was asked and couldn't have done anything more... This event is still being written about, and a high percentage of the fics are going "What the hell, Wilson?" in one capacity or another.
- From The Charmed fandom:
- Many Cole fans wrote accusation fics about how that Phoebe had flat out dumped Cole despite all the good he had done after he came back. Also, how that Phoebe never even bother to find out that Cole was possessed by the Source of all evil against his will. There's a fairly good fic called The Cleansing. It showed the direct side effects of what might happen.
- Pick any Mass Effect fan fic that deals with the aftermath of Horizon, particularly if Shepard romanced either Ashley or Kaidan. Chances are, 4 times out of 5, it will be a thinly-veiled accusation fic against the first game love interest (though it is worth noting that Kaidan gets the brunt of this more than Ashley does).
- The Avatar: The Last Airbender magnum opus Stormbenders was written by an author who believed Aang wasn't a true hero, that he was selfish and refused to grow up and change and thus painted him as an unsympathetic child. In her view it was Zuko who was the Real Hero (and more importantly, deserved Katara's love more).
- iCarly: Sam breaking up Carly and Freddie in season 3's iSaved Your Life by getting it into his head that Carly's feelings were not real. When Sam kissed Freddie in the season 4 finale some accusation fics were written about the idea that Sam's actions prior were because she was hiding her feelings but didn't want Freddie with Carly. Naturally when Carly puts two and two together in these fics they get dark.
- Quite a few fans of Veronica Mars were, to put it mildly, disappointed with the eponymous heroine's actions in season 3 (and, to a lesser extent, season 2, particularly its first half), thus fics where Veronica gets called on her attitude, especially concerning Logan, are pretty common. Still, it rather rarely goes dark, especially in Shipping cases.