I had a false positive for TNT a month or so ago in CLE. The guy shouted some code frantically, and another security droid came over, along with a supervisor who tried to explain to me what was going on in an extremely calm manner, as if to prevent me from using the triggering device. :-)

Of course, they retested things and sent me on my way, but it was a humorous false positive at the time -- everyone was very nervous as they recalibrated the machine.