People saw the plane hit the Pentagon, and it was captured on film. But then, nothing is ever good enough for a good conspiracy nut.
People also saw the 1st and 2nd planes hit the World Trade Center towers. Not as many with the first but many, many thousands of people with the 2nd plane. I guess they were all delusional, they all imagined hearing the jet airliner flying over head and all imagined seeing it crash into the tower.