How about a list of awful fanbases instead.
Sonic (I know, already said but worth mentioning again)
Serial Killers/School shooters. (needs no explanation.)
League of Legends. Fuck up in ranked match, and you'll never hear the end of it/you'll get banned.
Counter Strike Global Offensive (at least ranked people) Same as above, if you fuck up in a ranked match, they'll insult you to no end and outright kick/ban you.
Overwatch. Seems to be a bit different in that you get reported just by picking a certain hero lol.
https://us.battle.net/forums/en/overwatch/topic/20759236825
Essentially, the cases above are players who take the game way WAY too seriously.
Olicity shippers from Arrow.
Probably one of the more specific examples (and it's tied to the "people shipping real life people" fandom) but this one is so bad: the gist goes likes this, people liked to ship Oliver (main char) with Felicity (a supporting char). Around Season 3 and in particular Season 4, the writers started shipping them real bad, to the point Felicity debatably became the main character in S4 and she even dragged her family into the show. The problem is that the quality of the show went downhill as a result, not so much during S3 (the show started to go downhill due to other reasons not related to Olicity) but definitely during S4. It got so bad that the old Arrow fanbase (as in not the Olicity shippers) started leaving the show and r/arrow actually turned into a Daredevil subreddit in protest.
There's a youtube series of videos called "The Downfall of Arrow" which goes more in depth into the issues that plagued the show, and yes, it touches on the Olicity shippers.
The reason why I mentioned it was tied to the "people shipping real life people" fandom is because people legit want Amell (Oliver's actor) to divorce his current wife and marry Emily (Felicity's actress). They even photoshopped them together:
https://youtu.be/Fas7WVWJSSU?t=16m25s
Creepy stuff.