Character mistake: In the beginning scene where the shuttle lands on Pandora, as Jake gets on his wheelchair preparing to alight from the shuttle, the man behind him unloading equipment has his exopack mask loose even though he is exposed to Pandora's atmosphere. (00:05:20)
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.
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.