It is the extreme fans that get out of control and become the bad stereotypes that the average fan gets accused of being. Anne Rice has commented on how some of her fans freaked her out when they would come up to her at book signings with stange contact lenses and canine teeth surgically/dentally modified into "real" fangs.
Over the years I have been teased a bit about being a "Trekkie", but I only shake my head when I think about the football fans that paint faces and bodies, stand shirtless in snowy stadiums, let games monopolize their fall weekends, spend small fortunes on tickets, memorize their own set of "equally useless" trivia, and get in fights in bars over teams. The best was when my neighbor actually charged and kicked in his television set when his team lost a big game. His wife was less than impressed with him.