Sometimes they randomly open and close security lines between scanners and metal detectors. To avoid the hassle, I pick the lines that are obviously headed to a metal detector. When I opt out, I do it firmly but politely, and never had anyone say something except one time a TSA man said "You know this isn't dangerous radiation, right?" I just said "It doesn't matter. I'm opting out." I always take the pat-down in public and they vary a lot in quality.
I was surprised to find that leaving Tegel in Berlin was easy as can be, short lines, quick trip through the metal detector. The entry process at Sea-Tac was another story though. It took forever. You got grilled, you got the German Shepherd dog sniffing your crotch, and you had to wait for all your bags to be x-rayed. Some of the agents seemed to be behaving in a very rude way, in particular I remember one agent seemingly being a real jerk to some poor old guy who didn't speak English well. I remember thinking how bad an impression of the US that the foreign visitors in line must be forming. I was at that moment embarrassed to be a citizen.
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