"Infallible" defense and a false sense of security
The Trojans became incautious. The French failed to consider that Germany might think of Belgium as a thoroughfare rather than an inviolable sovereign nation. (BTW, Belgium's history may give people a hint concerning how its citizens feel about invasion of sovereign nations generally.) America was shocked that the oceans didn't protect us from attack in 1941. The Chinese never thought that the Mongols would figure out how to open a gate in the Great Wall.
People don't have to be idiots to believe in infallibility. They only have to be complacent. When one multiplies this by the gee-whiz factor that technology inspires in some people, we could develop an outstanding case of bulletproofbrain here.