(1) The hit point at the Pentagon was too small to accommodate a 100-ton airliner with a 125-foot wingspan and a tail that stands 44 feet above the ground; the kind and quantity of debris was wrong for a Boeing 757: there were no wings, no fuselage, no seats, no bodies, no luggage, no tail! Not even the engines were recovered, and they are practically indestructible.
(2) Of an estimate 84 videotapes of the crash, the three that have been released by the Pentagon do not show a Boeing 757 hitting the building, as even Bill O'Reilly admitted when one was shown on "The Factor". At 155 feet, the plane was more than twice as long as the 77-foot Pentagon is high and should have been visible. There are indications of a much smaller plane, but not a Boeing 757.
(3) Indeed, the aerodynamics of flight would have made the official trajectory--flying more than 500 mph barely above ground level--physically impossible, because of the accumulation of a massive pocket of compressed gas (air) beneath the fuselage; and if it had come it at an angle instead, it would have created a massive crater; but there is no crater and the official trajectory is impossible.
(4) Flying low enough to impact with the ground floor would have meant that the enormous engines were plowing the ground and creating massive furrows; but there are no massive furrows. The smooth, unblemished surface of the Pentagon lawn thus stands as a "smoking gun" proving the official trajectory cannot be sustained.