Continuity mistake: At the end, during the fight scenes in the first class cabin, the number of overhead compartment doors that are open changes.
Continuity mistake: United 93 is cleared to depart from runway 4L at Newark. However, you can see the terminals out the right side of the aircraft, indicating that it is departing to the southwest, on runway 22R.
Continuity mistake: After the hijackers have taken over the plane, the blond flight attendant's hair, styled in a French Twist, keeps changing from messy (presumably from the chaos) to as smooth and neat as when she boarded the plane. She's never offscreen long enough to have simply fixed it. Not until the passengers revolt does it remain disheveled.
Factual error: In one of the shots when the 757 is taxiing at Newark airport, you can see in the background an airliner in the new Air Canada colours. The current Air Canada livery was introduced in 2004.
Deliberate mistake: After takeoff, an exterior shot of the plane switches from a Boeing 757 to an Airbus A320.
Continuity mistake: When United 93 is in the air, the terrorist in the last row of the first class cabin has his brown jacket alternately zipped/unzipped between shots.
Continuity mistake: During the hijack, a female passenger goes to help another one who has been stabbed in the neck, bleeding profusely. When she returns to her seat, she has no blood on her.
Continuity mistake: In the Boston ATC sequence, the radar screen shows American 11 at a flight level of 350 (35,000 ft) this alternates with a flight level of 310 between shots.
Continuity mistake: Shortly after take off from Newark the plane has changed from a Boeing 757 to an Airbus plane as evidenced by the design of the winglets.
Continuity mistake: In both the exterior shots during the film and in real life, the Boeing 757-222 (N591UA) operated by United Airlines is shown to have 2 doors forward of the wing. In the interior shots, the front door is shown, but between the first class cabin and the economy class cabin, there is just a partition, no door.
Audio problem: When auditing the recording of one of the first hijacks, the FAA reviewer rotates the tape reels back and forth in order to find just the exact words from the hijacker. As the sounds play, the reel moves barely half a turn, but the reviewer rewinds it more than one revolution, not nearly returning to where that sound would be.
Factual error: During the take off roll, the curtain that divides First Class and Economy class is closed. This is in breach of FAA regulations that state the curtain must be stowed for taxi, take-off and landing.
Continuity mistake: In the beginning of the movie, when we see a hurried passenger trying to get through the security check, he takes his pocket change out of his pocket and puts it on top of the scanner. In the next shot, the money is gone and there's a purple tray instead.
Continuity mistake: Throughout the film if you look at the terrorist that wears the black polo shirt with the lines across it, the red bandanna keeps changing position, from thin and covered in hair, to thick with no hair covering.
Factual error: In a scene in New York's air traffic control tower (I think the one where they show the second plane hit the south tower of the WTC), there is a man wearing a black lanyard around his neck that says "Transportation Security Administration" on it. The TSA was formed shortly AFTER the events of 9/11 thus he should not be wearing it.