Character mistake: Just before the fire starts on the 81st floor, a man is shown telling an elderly couple that business offices only go as high as 80, and that 81-120 is exclusively residential. Not long after O'Hallorhan arrives, he asks Jernigan for a list of business tenants from 81-85, which Jernigan replies "most are yet to move in and those that have are not working at night". As said by the man earlier, these floors do not house business tenants, only residential. (00:12:05 - 00:43:45)
Suggested correction: Chief O'Halloran asked Doug for the floor plans from 81-85. He then asked Jernigan for a list of business tenants. Chief O'Halloran did not ask for businesses on any specific floor.
Character mistake: When the hijackers separate the lead car and back the rest of the train away, the conductor says in amazement, "I didn't know these things went backwards." Any subway conductor would know that trains run in either direction. Most trains relay (reverse direction) at their terminus, and any subway train can be operated from either end.
Character mistake: Near the beginning, Denise tells Stuart she has two small scenes in a movie, but then moments later tells him it is one whole scene.