According to Mendiburu (2009), 3-15% of the people can't view a stereoscopic 3D image or movie, mostly due to poor binocular vision.
Imagine one of those is in the movie theater, trying to watch a 3D movie. How would this movie appear to this person?
Mendiburu (2009) 3D Movie making: stereoscopic digital cinema from script to screen, Oxford, England: Focal Press) (Free PDF)