Back in Nov 2001, my wife managed to board a BA flight from Newcastle to Heathrow with an out of date passport after passing through 3 seperate security checkpoints, then passed through a further 2 checks at Gatwick before the BA staff at the boarding gate finally noticed the error (at the 6th attempt) and correctly refused her boarding the flight to Phoenix.
Given this took place a matter of weeks after 9/11 on a US bound flight when security was about as paranoid as it could possibly be, highlights that the system is far from infallible. Whether a deliberate attempt to breach security would have the same outcome is open to question.
Extremely embarrasing for both BA (and my wife, who should have checked her passport was valid some time before), who very kindly flew her out on the flight the folowing day FOC after she had purchased a new passport.