Being creatures limited by our instruments of perception, the behavior of the human eye and related processing systems never ceases to amaze me. Last year was particularly enlightening, as I hadn’t previously realized we effectively cannot see color in low light situations. I honestly never noticed until I woke up one morning and realized that I’d used the blue towel instead of the red towel after the previous evenings midnight shower.
Getting more heavily into photography has helped me understand the vast exposure corrections we make involuntarily and without consciously noticing. It’s simply not plausible to correctly determine proper exposure settings using the human eye alone. We adapt dynamically to different lighting situations, so your halogen lit living room at night–which seems like it’ll show well without a flash–will end up tragically underexposed if you “eyeball” the settings.
Perhaps my most interesting discovery of late is the checker shadow illusion: a great image demonstrating we often see what we’re expecting, rather than what actually exists. But then, “what actually exists” is a different issue entirely.