NORAD's Northeast Air Defense Sector (NEADS) held a training exercise just over a month before September 11, 2001, which had some uncanny similarities to the 9/11 attacks. The exercise, called Fertile Rice, was based around the scenario of Osama bin Laden--the man who supposedly ordered the 9/11 attacks--organizing an aerial attack on a high-profile government building in Washington, DC--one of the cities attacked on September 11.
NEADS personnel were scheduled to take part in an exercise on September 11. We therefore need to consider whether the similarities between the scenario for the Fertile Rice exercise and some of the incidents they had to deal with on the morning of September 11 caused them to mistake real-world events for part of the day's exercise and thereby impaired their ability to respond to the 9/11 attacks.
EXERCISE INVOLVED BIN LADEN PLANNING TO ATTACK WASHINGTON WITH A DRONE AIRCRAFT
NEADS, based in Rome, New York, was responsible for monitoring and defending the airspace in which the hijackings occurred on September 11, and was consequently responsible for coordinating the U.S. military's response to the 9/11 attacks.  It ran an exercise called Fertile Rice each week.  On August 4, 2001--five and a half weeks before 9/11--Fertile Rice was based around the scenario of Osama bin Laden's operatives attacking a target in Washington.