From the Washington Post:

The Transportation Security Administration promotes its programs to ensure security by using undercover operatives to test its airport screeners. In one instance, however, the agency thwarted such a test by alerting screeners across the country that it was under way, even providing descriptions of the undercover agents.

The government routinely runs covert tests at airports to ensure that security measures are sufficient to stop a terrorist from bringing something dangerous onto an airplane. Alerting screeners to an undercover officer's timing and appearance would defeat the purpose.

But that's exactly what happened on April 28, 2006, according to an e-mail from a top TSA official who oversees security operations.