"If a dry run is occurring, the attack will shortly follow."

That's according to Wolf Koch, a Boeing 767 pilot for Delta Airlines and an official at the Air Line Pilots Association International, who was discussing common terrorist tactics in the run-up to devastating attacks like those on 9/11. That is an extremely worrying thought given a memo leaked this week from the U.S. Airline Pilots Association, which warns of "several cases recently…of what appear to be probes, or dry runs" of such attacks.

The most recent dry run may have occurred just weeks ago, on September 2, on board flight 1880 from Reagan National Airport in Washington, D.C., to Orlando International. According to the memo:

A group of Middle Eastern males boarded in DCA. Shortly after takeoff, one got up and ran from his seat in coach towards the flight deck door. He made a hard left and entered the forward lav, where he stayed for a considerable length of time! While he was in there, the others got up and proceeded to move about the cabin, changing seats, opening overhead bins, and generally making a scene. They appeared to be trying to occupy and distract the flight attendants.

It's common practice for potential terrorists to carry out dress rehearsals of their planned attacks, and most major terror strikes of the last few decades have involved a dummy-run of some sort. Some have even accidentally involved celebrities.

Before al Qaeda terrorists hijacked four commercial planes and crashed them into the World Trade Center, the Pentagon, and a field in western Pennsylvania, killing almost 3,000 people, they carried out a number of dry runs — including one on a flight carrying Oscar-nominated actor James Woods. Four of the future attackers were apparently sitting in first class with Woods on the Boston to Los Angeles flight, and behaved so strangely — sitting erect in their seats and staring ahead for the whole flight — that the veteran Hollywood star called the FBI the day after 9/11 to report his unsettling experience.

If that wasn't bizarre enough, Rob Lowe also revealed shortly after the death of Osama bin Laden that he, too, had been on one of the terrorists' test flights:

Similarly, in the 7/7 attack in London in 2005, the four attackers carried out a reconnaissance mission two weeks before detonating suicide bombs on the London underground, killing 52 people. The dry runs at Luton station and London's King's Cross were caught on security cameras (from around 0:50 onwards):

The TSA has said that it investigated September's unsettling incident thoroughly, and that no further action was required. Some are now speculating that it may have been a test by officials to check that the correct security procedures were in place.

All the same, the memo urges pilots to stay vigilant:

"Don't be 'the one' who lets the Company pressure you into taking a flight you are not comfortable with," it reads.

"Fly Safe — Fly Alert — Fly Informed."