Other mistake: When Michelle Rodriguez is flying the copter she doesn't have a mask on when they are at the tree of life to shoot down the tree. The machine gunner has his mask on. It's an open cockpit.
Factual error: During the hometree bombing, the "sky people" throw gas grenades at the N'avi. It's a cool tactic but because of the helicopter blades rotating right above them, the gas would just get blown away therefore making it a useless thing to use in the war.
Other mistake: During the battle between the marines and the Navi, colonel Quaritch shoots down Trudy's ship. Despite her cockpit having bullet holes in it, somehow she is still able to breathe. Unlike Quaritch when Jake sticks the knife into his AMP.
Continuity mistake: During Jake's first video log, Dr. Spellman and Dr. Patel are leaning on a table. When the camera changes and Jake turns his head over his shoulder to ask the two doctors a question, Dr. Patel is much more leaned over than before the camera change. (00:13:48)
Chosen answer: The Core didn't originate the name - it's been used since the 50's and even has its own Wikipedia article at http://en.wikipedia.org/wiki/Unobtainium. There it's described as "any fictional, extremely rare, costly, or impossible material, or (less commonly) device needed to fulfill a given design for a given application."
Jon Sandys ★