When they turn on people not involved and innocent people get hurt. Some of that is an unavoidable consequence of mob mentality, but more specifically this article talked about the history of 1980's nazi's v.s. the trad skin heads. Most people don't know that skin head wasn't a racist thing before the nazi's appropriated the look and culture, shit even the phrase rude boy isn't inherently racist until neo-nazi's started using it. Just look at the specials. It caused all sort of things to spring up in its wake like lace codes, specific brace colors, how many eye docks was considered ok to wear. It was basically gang warfare that was never resolved. As a "former" punk rocker (aka I don't have my mohawk any more and dress like a nerd for the job), these codes can easily be broken by people who are not in the know and simply dressed in paramilitary gear because they would be covering a protest as a reporter. You just don't know because these things happen so fast, but as soon as someone feels confidence, every one feels confidence and assumes the worst.