Last flight last year CO - IAH - LAX
Boarded with F/C pre-board then all boarding stopped, All passangers and flight crew had to leave plane with all carry-ons to stand in Jetway while plane was re-screened.
Explanation from Continental (friendly F/A who I regularly see in F/C) said that a randomly selected indivisual had boarded without being randomly searched.
I am male, fly alone, and often fly with just a small carry on since I have clothes at both ends. Never been randomly selected.
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"A Southwest line never gets longer, It only gets wider"