I have run-ins with cops and security guards while photographing all the time. Often I'm told I'm not allowed to be doing what I'm doing, even though I have every right in my favor. Because I am usually unafraid of confrontation, I challenge these people. I have yet to lose (except for those times when I'm on private property and am simply playing dumb). While shooting one of Bank of America's buildings I had a cop try to lie to me to get me to go away (he was being paid off-duty to guard the front of the Bank of America corporate headquarters while in uniform), but when I pointed out all the right in my favor he responded, "Yeah, you're probably right, I don't work uptown much and don't really know the rules." Urgh! Anyway, because of all of the police v. people protesting happening in Ferguson lately Wired did an article about police arresting journalists for doing their constitutionally-protected jobs. Urgh again! Anyway, check out the article, it's good.... http://www.wired.com/2014/08/why-and-how-we-must-protect-the-right-to-film-cops-in-ferguson/
P.S. I appreciate cops most of the time.