"We didn't start the flame warbehave in ways that they wouldn't when they're face-to-face with other people. What starts as a minor disagreement like "how to ship two minor characters," or "what constitutes canon or fanon," escalates until the users are insulting each other, flinging expletives and profanities and shouting at the top of their virtual lungs. Inevitably, someone will be compared to Hitler. After a time, the flames may abate, but usually more because of exhaustion than anyone actually getting a point across. Once everyone has recovered their stamina, all it takes is one innocent (or not-so-innocent) person bringing the subject up to start things all over again. In fandom related communities, a lot of the flame wars are the result of a Broken Base, Fandom Rivalry, Fandom Heresy, or other Fan Dumb behavior — which, in turn, is the result of people regarding the fandom as Serious Business. In a way, it can be even more bothersome than dealing with trolls who come onto the community to bash the entire fandom. Sometimes a fandom Flame War can reach such levels that it becomes a Holy War, a massive, prolonged battle of insults. The Console Wars and PC vs. Mac debate are good examples of these prolonged, though rare, wars. God help you if you actually start discussing politics or religion. See Misplaced Nationalism for a specific form. See also Internet Backdraft.
Particularly common causes:
Anyone got popcorn?