Among Fandom there are some topics which, once mentioned, will cause endless, passionate debate over which faction of the Fandom is correct. Then there are some things which it seems like everyone in the Fandom agrees on. Of course Alan Moore is one of the best comics scribes out there. Of course the American adaptation of Coupling sucked. And of course Garfield has been going downhill for years. How could anyone think otherwise?
Well, inevitably there are one or two fans who do. Actually voicing these opinions which run contrary to Fandom consensus, however, can entail some risk. You might get some funny looks if you said you preferred the Japanese GoLion over the American Voltron Lion Force. If you say you liked Voltron Vehicle Force, however, prepare to be treated as a Fandom pariah, along with comments that you're obviously not a real/true fan. Essentially, this is the Fandom variant of The Complainer Is Always Wrong.
Sometimes, this trope can even extend to fans who hold opinions that are only tenuously related (or, in some case, entirely unrelated) to the work in question - which run contrary to Fandom consensus. Can exist in the form of "you can't be a true fan of Band X, if you also like Band Y or you don't also like Band Z".
By the extension of the above two paragraphs, in certain circles, the term "true fan" is the fandom equivalent of Godwin's Law. It's like declaring "I believe in X because I'm a 'regular' fan". In other words, if someone plays/invokes the "X because I am a true fan" , then they automatically lose the argument.
Please note, to count as Fandom Heresy it must be said by someone who is part of the Fandom. For example, someone saying "Anime is stupid" or "Discworld is boring" doesn't count. However, an anime fan saying, "Hayao Miyazaki sucks" or a Discworld fan saying, "Sam Vimes is the lamest character in the series" would definitely count.
A very commonplace, fandom-independent example of Fandom Heresy is disliking the main character, or the Ensemble Dark Horse if the main character isn't also the most popular character. Doubly so if you say that said character is a copy of a character from another series. Being a fan of The Scrappy is just as universal an example of Fandom Heresy, for equally obvious reasons.
See also Fan Dumb for another way to hit the Berserk Button of certain fandoms. Compare with Broken Base, where the two sides of opinion are more equal rather than one overwhelming majority against a minority "heresy"; as well as Sacred Cow or Unacceptable Targets, in which the "heresy" includes even non fans. Not to be confused with Fandom-Enraging Misconception, which is for common factual errors about works that particularly annoy their fans.
No examples, please. What might be considered to be something "everyone" within a fandom is supposed to like often relies on personal opinion which can lead to heated debates, and we're not really too keen on having that here.