The same sort of thing as what is being described in Texas seems to have occurred on February 10, 2008 in Virginia Beach when Obama spoke to roughly 18,000 people at the city’s convention center. The Secret Service had a totally inadequate number of screening machines to handle the crowd (about five or six machines total).
Thus, after screening the people that were allowed in closer to Obama, a back section – which was cordoned off – was filled with the majority of the crowd none of whom were screened. The crowd just surged in through the rear doors where no screening machines were located. It definitely left me a bit unnerved.