Anime Boston
In 2010 (I did not attend that year) a bunch of Hetalia cosplayers (of the Germany character)
did a Hitler salute on photo. I wasn't there. The kicker: This was during Passover apparently, and it was near a Holocaust memorial.
In 2012, some shit went down at the "informal dance" (this is the way they sugarcoat the word "rave") - someone had pot, it was overcrowded (fire safety hazard) and some guy was attacking EMTs. I wasn't at the rave.
Here's an article about it. Since then they removed the rave forever a lot of people got really upset. Too bad.
In 2013 I was on my laptop in the hotel lobby talking to friends on Steam. There were convention attendees in line to get their hotel rooms, a whole load of them. Suddenly, a REALLY REALLY REALLY HUGE guy comes in from the doors of the Sheraton's front, and sees his REALLY REALLY REALLY REALLY HUGE friend in line. He calls out at he top of his lungs his friend's name, forgot what it was. They charge at one another and "glomp" (tackle hug) each other, frightening a few of the anime people and the hotel staff, and knocking over some rope partitions. Some girl yells "THIS IS WHY WE CAN'T HAVE NICE THINGS!"
This year I was dressed as Clover from Payday 2. Some guys complimented me, and I said thanks. I then noticed them talking then looking at me. One of them said "he thinks you're cute." Note Clover is a Payday 2 character. All except one are usually masked. I pointed out they don't know what I look like under this mask, and walked away.
I've seen some cringeworthy stuff sold at the Artist Alley. Things like "Demisexual pride" scarves, or "SJW" "Tumblr famous" buttons and all that. Most of the stuff in the artist alley is usually laminated prints of fan art or buttons, sometimes acrylic charms for phones or whatever.
Anime Weekend Atlanta
I only went in 2013 for Touhou shit. People there were decent for the most part, but someone stole some merchandise that was expensive, and during a Q&A with the guests from Japan, an unwashed nerd tried to talk to the Japanese people in broken Japanese. Nobody knew wtf he was saying. He then said his comments in English "YOU GUYS ARE LIKE GODS TO US!"
There was also a guy in the Dealer's Room selling Yaoi Nazi bondage gear. Links to a different convention forum but same guy.
SOE Live (The EverQuest convention)
You know, the funny thing is, is that I literally had no problem at this convention when I went in 2014. Was mostly older nerds there. The only fedoras I've seen were on middle aged women. The developers also talked to me a lot. Cool people. Not that the other conventions weren't fun, I mean, if I didn't have fun at these things I wouldn't go.
I heard that one year at the EQ con someone shit on a hotel bed, took pictures of it and posted it all over the forums.
AnimeNEXT
Oh man.
I wore a cosplay that revealed my midriff. I was a Green Lantern. I was hit on so many times by nerds, like "in brightest day babe" and grabbing my ass and stuff. I was propositioned for sex twice. Some sentient pile of lard dressed as a pony gave my friend's girlfriend a dirty look because she was Lilith the succubus (Morrigan's sister) and she's fit. A few guys took my photo without my permission. It was also overcrowded, but I did get to hang out with my friends and finally met some EQ people I knew for years, so that was nice.