At LaGuardia, a man successfully walked through the metal detector, but screeners wanted to check his shoes. (Some reports say that his shoes set off an alarm.) But he didn't wait, and disappeared into the crowd.
The entire Delta Airlines terminal had to be evacuated, and between 2,500 and 3,000 people had to be rescreened. I'm sure the resultant flight delays rippled through the entire system.
Security systems can fail in two ways. They can fail to defend against an attack. And they can fail when there is no attack to defend. The latter failure is often more important, because false alarms are more common than real attacks.
Aside from the obvious security failure — how did this person manage to disappear into the crowd, anyway — it's painfully obvious that the overall security system did not fail well. Well-designed security systems fail gracefully, without affecting the entire airport terminal. That the only thing the TSA could do after the failure was evacuate the entire terminal and rescreen everyone is a testament to how badly designed the security system is. [Schneier on Security]