A couple years ago this happened at Tufts University. Someone called the campus PD and said they saw a guy walking down the street with a revolver. So security alert emails went out told everyone to stay inside. Eventually the guy called the PD and said that he fit the description and was walking down the street twirling a socket wrench at the time the call came in. No gun, just a paranoid hippy that can't tell the difference between a socket wrench and a revolver.
Of course this launched a campaign around the campus saying that the only reason the caller thought it was a gun was that the person with the wrench was black. The flyers were hilarious.