On August 22, 2002, the Associated Press ran a story about 9/11. "Agency planned drill for plane crash last Sept. 11."
"...one US intelligence agency [NRO, National Reconnaissance Office] was planning an exercise last Sept. 11  in which an errant aircraft crashed into one of its buildings."
The same morning. As. The 9/11 attacks.
Accrding to the NRO, their exercise was canceled when the real thing began.
Barbara Honegger, who worked in the White House under Reagan, points out another coincidence. Rsearching press reports, she found a 9/16/01 Washington Post story about the pilot of AA flight 77 that, on the morning of 9/11, was said to have crashed into the Pentagon.
The pilot, Charles Burlingame, an ex- F4 Navy flyer, had, as his last Navy mission, helped craft Pentagon response plans in the event of a commercial ariliner hitting the Pentagon.
Pilot drafts plan for response to Pentagon hit. Pilot winds up on plane that hits Pentagon.
Honegger states that Dick Cheney was ultimately in charge of the NRO exercise on the morning of 9/11. He was in the White House Situation Room for that purpose.
How do you like all these apples?
The limited hangout on this would be: "The hijackers had found out about the upcoming 9/11 mock exercise. They ran their op on top of that, hoping the confusion between Real and Mock would keep the US government from responding to the actual attacks."
Or, one could take this another step: NRO uses many CIA employees. Some element of the CIA was involved in the tactics of the actual 9/11 attacks.
All the above coincidences certainly defy the laws of probability.
Since AP eventually ran a story right out in the open about the mock exercise, one would think the Hill would have exploded in outrage. A hearing would have been held pronto. The "bizarre coincidence" would have become front-page news for a week or so.