United Airlines personnel were subjected to a surprise training exercise 12 days before 9/11 in which they were led to believe that one of their planes had crashed. The exercise was so realistic that some of them ended up in tears or became physically sick. Consequently, on September 11, 2001, when two United Airlines planes were hijacked and then crashed, the manager who organized the exercise apparently thought his employees had mistaken reports about the terrorist attacks for part of an exercise and therefore told them, "This is not a drill!"

Furthermore, United Airlines had previously conducted other exercises that were based around scenarios resembling aspects of the 9/11 attacks, which may have caused its employees to be confused on September 11 over whether the crisis that day was real or simulated. The scenarios included hijackings and planes crashing into buildings.