Fans being: opinionated, argumentative, rude, touchy, ignorant, wanky, whiny, elitist, irrational, or any combination of the above. In short, fans behaving foolishly. Often results in fanwank, flamewars, and trolling.
The wild uproar from fans of particular subject matter or a public figure when anything is done with the subject matter or public figure regardless of the quality of what is being done to, with or by the subject matter or public figure.
When Hollywood doesn't keep to the source material for movies based on comic books, fandumbmoanium breaks loose!
Everytime anything to do with Twilight happens, every household with a female in it turns into complete fandumnmoanium.
A type of fandom that isn't involved in the internet community except to constantly deride the subject matter, especially the newer examples. Usually an old school purist. Especially common in anime and Final Fantasy fandom.