NBC News 
June 8, 2013
The National Security Agency has at times mistakenly intercepted the private email messages and phone calls of Americans who had no link to terrorism, requiring Justice Department officials to report the errors to a secret national security court and destroy the data, according to two former U.S. intelligence officials.
At least some of the phone calls and emails were pulled from among the hundreds of millions stored by telecommunications companies as part of an NSA surveillance program. James Clapper, the director of national intelligence, Thursday night publicly acknowledged  what he called “a sensitive intelligence collection program” after its existence was disclosed by the Guardian newspaper.
Ret. Adm. Dennis Blair, who served as President Obama’s DNI in 2009 and 2010, told NBC News that, in one instance in 2009, analysts entered a phone number into agency computers and “put one digit wrong,” and mined a large volume of information about Americans with no connection to terror. The matter was reported to the Foreign Intelligence Surveillance Court, whose judges required that all the data be destroyed, he said.
Another former senior official, who asked not to be identified, confirmed Blair’s recollection and said the incident created serious problems for the Justice Department, which represents the NSA before the federal judges on the secret court.