I've not specifically encountered any of this (outside of some internet arguments) but I have an intertesting viewpoint to share...
Some series/movies/manga/etc dont have a fandom so much as a hatedom (or in some cases they have a large base of both) that is everyone who knows the work hates it. As mentioned above, the Twilight books/movies make a good case here: outside of their target demographic, almost everyone hates them (speeking in very general terms here). So you should know what you could be getting into before you get into it. Likewise some fandoms have a broken base where the series changed (or some fans think it should have changed) and part of the fandom likes it and part hates it (often happens when an anime follows the manga upto a point then diverges) While you can never know for sure which slice of the fandom you'll find yourself facing at a con, it's again good to be aware of massive rifts...
on a tangentially-related not it's disheartening to hear people saying the bronies are turning into idiots, given the motto "love and tolerance" (even if 2/3 of the fandom doesnt follow it) at least it looks like it's still minor enough that I dont have to worry about any troubles from my RD cosplay... (right?)
'Bring peace to the lost lamb, grant rest to the fangs of the wolf, and deliver the Hammer of God unto the Devil', - Sister Rosette Christopher, Chrno Crusade
"The bad news is he ended up building the whole rock tower... the good news *gulp* I have a rock tower to knock down..." - Rainbow Dash, My Little Pony: Fiendship is Magic (S03E09)