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)
Chosen answer: When Ra saw the bomb, he realised that it was a few seconds away from detonation. There wouldn't have been enough time to send it back.