Fans of most things really. I mean, I like a lot of things. I would say I was a fan of a great many of them. But there are points when people take fandom far too far.
Little kids who think they're amazing because they're allowed to play CoD. People who only ever play CoD and think they're some kind of 1337 gaming Gods because they pulled off a cross-the-map-360-no-scope-wallbang-quad-headshot or something ridiculous like that.
People who are against CoD in every way and think that people having the sheer gall to play it makes you some kind of inferior life form, and through the wonders of Ad Hominem use this to decry every comment you ever make about anything as being false or invalid.
The Halo fans who pretty much act exactly like that Awesome Reach video that somebody posted earlier.
The Twilight fans who won't shut up about how it's the best thing ever. Fair enough, Twilight was good, New Moon was shit and Eclipse was ok, but still. I haven't read Breaking Dawn (yes read... like books) but I think I've read enough to realise that it's not the best thing to ever grace this planet.
Console fanboys. Enough said. Just look at any forum or thread where they engage in the console wars and you'll see some of them get pretty OTT.
Fans of particular genres of music can really get annoying too. The chav scumbags who think they're the be all and end all and think they're going to be the next great MC. The chindies who are really chav scumbags but are in denial, and they went to Evolution and went in all the 'Mosh Pits' and think they're hardcore. They often engage in 'Mosh Pitting' which doesn't even exist. When things get a bit hectic in school corridors they say "Hew this is like a mosh pit! YESSAAAAIIII!" No. This is just people stupidly close together moving forward quite fast. I'd love to see the 'Mosh Pits' you've been in. I'd probably get stupidly bored and start a proper one with some friends. Then there's the Justin Bieber fans. Ugh. Just go on Youtube and search for something like, Justin Bieber vs. Metalheads or something like that. It's actually ridiculous.
Some random annoying kid with a lisp: "This is a message to all Justin Bieber haters. I am here to tell you that you all are wrong. You all like heavy metal or something instead of decent music from people with actual talent. Well, metal sucks dick. This is an open declaration of war on all 'metalheads',". Oh dear. He goes on and on and on for aaaaaages, and it's all absolutely ridiculous. There are many more videos of a similar nature. There are also very serious, well-thought out arguments from the opposing side. It's all ridiculous.
Then there are the teenies, and the fans of bands like Black Veil Brides. The people who will blindly follow bands anywhere, supporting any change at all in style or musical direction, simply because they think one or more of the band members (usually the singer) is 'fit'. These people sicken me sometimes.
All in all, it's just fanbases really. If something creates a fanbase, it's more than likely that someone will blow it ridiculously out of proportion, and take things way too far.