On and shortly after 9-11-01, supposition was that up to 10 flights may have been targetted and had terrorist groups onboard. For various reasons, they were not all able to accomplish their missions. Some speculated or had info that planes taking off on both the east coast and west coast were targetted for hijacking.
IIRC, Flight 93 had a delayed take-off. That was what allowed those passengers to later find out that attacks were underway.
There was a group of ME men who missed a plane in San Antonio (IIRC), and took Amtrack to St. Louis.
We have no idea how many actual terrorist plots were supposed to take place that day.
The original plot, sized down by bin Laden, had planes also in Europe/England being hijacked. That part apparently was cancelled.
That's the one I'm thinking of. According to a passenger we met in Big Sur (the flight was bound for San Francisco), they did get on the flight. It was delayed because of a cargo net (in reality, the pilot probably held it up for his own reasons) and the ME men forced their way off the plane and took off.