
Visible crew/equipment: Throughout the movie, in the shots of the guys on the alien planet, the white reflector screens and film crew's reflections are visible in their sunglasses. (00:40:20)

Continuity mistake: When Jackson is in the meeting about the symbols, he circles the seventh symbol with a blue marker. In the next shot, look close and you can see that the blue circle around the symbol is gone. (00:18:20)
Answer: A human uprising forced Ra to leave Earth thousands of years ago. He likely fears a second one could actually destroy him.
Captain Defenestrator