Seen this before
This is interesting. A month or so before 9/11 I worked as a F/A for USAirways, and I'm pretty sure we had a dry run on one of our flights, or at least they were testing security to see if we took it seriously.
I was lead F/A on a trip and our first leg was PIT/MIA, although it was a through flt that had originated in EWR. (I believe two of the 9/11 flights left from EWR and at least some of the terrorists learned to fly in MIA.) Well there was a crew change at PIT - we were to take the PIT/MIA leg, but two middle eastern gentlemen had thru checked bags at EWR, got off at PIT and not reboarded. Thankfully our captain deemed this a security risk and would not take the flight unless they showed up, or all the baggage was unloaded and rescreened.
The ground staff looked further into the missing people and discovered that two of three red flags had been met. (It's twelve years ago now, and I can't remember exactly the criteria but I think they'd paid for them in cash, they were one-way tickets but they'd been bought more than three weeks ago - I think that if they'd been bought more recently then all three would have been raised, but don't quote me on that.)
Anyway this was my second year in the US - before that I'd worked on the ground for BA for twelve years, where we took security extremely seriously! These were the pre 9/11 days in America though, so there was still some discussion amongst the ground staff as to whether we should be taking the flight or not. The captain stood his ground, and the chief pilot came down to the gate to talk to him. I called my manager and explained what was going on, and was told that as long as I could back up my decision with the facts I was telling him that I'd have his backing. Because the captain was still standing his ground the chief pilot asked the flight crew to leave the aircraft and they were to get a new crew. He then asked me if I was also refusing to fly! I responded "I'm not refusing to fly, I'm following the direction of my PIC." The flight attendants were also replaced.
We went back to the crew room where we filled out reports and heard nothing more about the incident, until 9/11.
On my first trip back after 9/11 there were obviously a lot of "people in authority" around offering assistance to F/A's. I told one of them about what had happened, and he asked me to speak to an FBI officer, and relay the story to him.
He said that this was the second time he'd heard of something like this happening - the other time was almost identical, and had involved a flight into Houston, which of course was the other place where some of them learned to fly.