If you look at soccer hooligans in Europe, I would say yes.
The way things are today over there, gangs organise fights at the games which have led to many people being killed or seriously injured.
They disguise themselves as genuine suppporters but are only threre for one reason and thats to instigate violence amongst the crowds, usual law abiding people get mix up in these fights and end up regretting it for the rest of there lives assuming they live through it of course, the guys who started it all vary rarely get arrested or hurt as they step away from the scene once they achieved there objective.
I watched a documentery on it once where a guy went undercover with one of these gangs and once at a game they Dispearse amongst the real fans, once they decide the time is right they contact each other on cell phones and start the trouble, usually by throwing whatever they can at the supporters of the opposite side, sad but true
