Continuity mistake: When Sigourney Weaver is shot in the stomach, she is not wearing make-up. When she is transported to the Tree of Life and laid under it for possible rejuvenation, she is wearing mascara.
Continuity mistake: When Quaritch is scolding Jake for making out with a Na'vi, Jake moves his head to the right. In the immediate next angle, his head is straight. (01:31:25)
Continuity mistake: When Jake is learning to ride on the direhorse, after he is corrected when calling she a he, and is hit on the head by the direhorse's queue, his braid slips over his right shoulder, but next shot the braid has returned to his back.
Continuity mistake: At approximately 15 mins and 15 seconds, Max is looking over the image and info on the activity for the Avatar drivers. When Jake first drives, he is wearing a black "dog tag" with a blue spot. When he walks over toward the viewing window with the portable screen at approximately 15 mins 30 seconds, the "dog tag" is orange with a spot of black. (00:15:15)
Deliberate mistake: When Selfridge says "Pull the plug" after Hometree is destroyed, two soldiers, one male and one female, go to shut off Jake and Grace's avatar links. The female soldier goes straight to Grace's link and terminates it, but the male soldier gets delayed (punched by Norm) before he can terminate Jake's link. This is deliberately shown out of sequence - we see Jake's link terminated before Grace's. The result is that we see Grace's avatar fall unconscious when Jake's link is terminated, and Jake's avatar fall unconscious when Grace's link is terminated.
Character mistake: Grace is doing something 'scientific' in the background while Jake is submitting his video report. She holds a filled pipette upside down, which caused some mirth with the biologists watching the movie with me. The pipette is always held with the opening downwards, because if you turn it upside-down, liquid may flow into the pipette mechanism and contaminate it, or become contaminated. Grace however turned the pipette with opening upwards, which is not what a professional scientist would do.
Continuity mistake: In the scene where Grace is talking about the Tree of Souls in the mobile lab, Trudy stands behind Jake with her right hand on her hip and left hand hanging down. After grace says "Outsiders are strictly forbidden," we are shown a close-up shot of Jake and hear Grace continue with "You lucky swine." In the background, we see that Trudy has suddenly turned 90 degrees clockwise and is apparently supporting herself with her left arm on what seems to be the same table Norm is sitting on. (01:12:00)
Continuity mistake: In the scene where Neytiri and Jake are standing under the Tree of Voices and Neytiri says to Jake, "This is a place for prayers to be heard", Neytiri's queue is at her back. In the next shot it is hanging over her right shoulder. (01:18:45)
Continuity mistake: When Jake is recording his first video log entry, he moves the recording camera around a little to get a better view of his face, and is looking directly at a screen to his right to see if he's centered his face correctly. In the very next shot, he is staring straight forward at the camera. (00:10:10)
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.
Continuity mistake: In the scene shortly after Neytiri saves Jakes life in their first encounter and Neytiri knocks Jake down she is holding her bow with arrows. When the shot changes the arrows are gone, and when the shot changes again they are back. (00:44:00)
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)
Continuity mistake: When Jake is trying to catch up with Neytiri, they get ambushed by Tsu'tey and his posse. Jake pulls out his dagger, but then throws it down to save his butt. He's surrounded. So he has his arms out where they can see it. While Neytiri is confronting Tsu'tey, Jake is in the background with his arms spread out, and his dagger back in the holder. (00:42:10)
Visible crew/equipment: When Trudy is firing up the gunship when helping Jake and co escaping from the Company HQ, a microphone is visible under Trudy's tank top, near her right breast.
Chosen answer: Jake and Neytiri were holding hands and Neytiri had her hair unbraided. It is also possible that when Na'vi mate, their appearance changes slightly in a way that only other Na'vi could see.