Here a quite surprising media account:
Two of the hijackers had bought tickets for flights scheduled after the
Sept. 11 attacks. Ahmed Alghamdi, who was on the United Airlines plane
that hit the World Trade Center, had also purchased tickets for a flight
the next day from Dulles Airport in Washington, D.C., to Saudi Arabia.
Atta also appears to have been booked on a Delta flight from Baltimore
to San Francisco; the date of that flight is unclear. (Hartfort Courant, 9/19/01)
And also the Senate Report mentiones this story (although with slight differences)
Hamza Al-Ghamdi purchased his ticket for Flight 175
through the Internet on August 29, 2001, using his Florida SunTrust Visa
debit card. He also purchased an E-ticket for United Airlines Flight
7950, flying from Los Angeles to San Francisco, at the same time. (Statement for the Record, 25. 9. 2002, FBI 24003, p. 8)
http://intelligence.senate.gov/0209hrg/020926/mueller.pdf Hm, rather strange.
But in fact it can be always useful to book flights for the time after 911 just in case one misses the Portland Flight and comes too late to Boston to hijack AA 11 ....