The dumbest incident I heard of was a guy who was looking a a .22 pistol. Since it was a rimfire and he didn't want to damage the gun by dryfiring it, he somehow got it into his head that the proper course of action was to chamber a live round into it and fire it at the floor -- right in the middle of the busy gun show.
Needless to say, his willingness to go the extra mile to preserve the firing pin for future generations was not greatly appreciated by the dealer, the patrons, or the uniformed officers providing security for the event.
'long about ten years ago, some butthead (skinhead neo-nazi looking kind of guy) tried to steal a pistol from a dealer at a Baltimore, MD area gun show. That dealer, two neighbouring dealers, and some attendees tackled him, cuffed him, roughed him up a bit, and held him face down (REALLY down ;'} ) on the floor while the MD State Police made their way over to formally arrest him.