He was dressed as a zombie from Resident Evil's Umbrella Corporation, complete with gas mask, grenade, machine gun, all that good (and fake) stuff. But it was so convincing, apparently, that when he walked into a store (Metro, who I can totally imagine having zombie problems), someone called the police thinking he was a real masked gunman.
A dozen cops ran into the store with their guns drawn not much later and arrested him, though even the store owner was aware that he was in costume. Apparently they weren't aware of the event, which had also happened the weekend prior without a hitch.
Don't go getting too upset though. How often do we hear people complaining about how no one bats an eye at stuff that's possibly really bad going down anymore? He was the first to arrive in costume, so it's not like there were dozens of others to make the situation obvious yet. And the cops detained him only briefly, presumably to ensure that he was not, in fact, dangerous. And the man scored top prize at the crawl in the end, so he gets a VIP pass to the convention.
So all's well that ends well. But convention organizers? It never hurts to give the police a heads-up that events like this are happening! I actually remember sitting in while a bunch of my friends discussed their airsoft battles and how important it was to ensure that the local police were made aware of their plans, because it only takes one person to see a bunch of people in masks running around shooting each other at an otherwise abandoned location to ruin the party.
What do you think? If you didn't know a cosplay event was going on and you saw someone with what appeared to be a gun and a mask heading into a store, would you call the police?