Factual error: The flight attendants didn't do the safety briefing properly. None of the flight attendants show the demonstrations. Also, the safety briefing should be given during taxiing to the runway. But in the movie, the flight attendant had finished the safety briefing before the door of the plane closed. During the safety briefing, the flight attendant said that the aircraft has 6 emergency exits. How come a 747 has only 6 exits? A 747 in fact has 5 main deck exits on each side, which means on the main deck there are 10 exits. Judging by the picture of the plane on the DVD inner cover (New Line Platinum Series), the 747 used in the movie most likely is the 747-200 series, which means the upper deck has one emergency exits. All in makes the plane has 11 exits, almost double the number said by the flight attendant. And during the take off, when Alex looks outside the plane to the wing, he sees that the flaps are just being deployed. When a plane is taking off, the pilot needs to make sure that the flaps are deployed BEFORE they are actually on the runway. (00:10:05 - 00:17:00)
Other mistake: When the plane takes off in Alex's vision, the plane begins to go into a nosedive, and the entire left fuselage is blown apart, sucking many people out of the plane. The plane then explodes completely, but in reality, the plane just takes off, and explodes almost instantly afterwards. No struggling with the plane is shown in reality, nor is the plane going into a nosedive. This is also proven in FD5, which is a prequel. The jet engine explodes, leaving a flaming tail, and the plane spinning out of control. The plane just simply "explodes" without any signs of struggle.
Other mistake: When Carter is basically trying to kill himself with the remaining "survivors" in his car, Billy makes a comment saying that when he saw Alex leaving Mrs. Lewton's place, Alex's shoe prints were in her blood and his prints were on the knife. All good and fine but there's just one thing that doesn't make much sense. Her house blew up right after he ran out, so could anyone still pull off any prints from the knife that killed her, and see footprints by her body? Wouldn't that evidence be gone in the explosion?