When I walk across campus, I am always looking around at the surroundings and other people. I like to be in the habit of maintaining good situational awareness, and it is very rare for anything to surprise me. A couple days ago, I was mildly surprised when I suddenly became aware of a person about twenty feet away that I had not seen before. It was a cadet wearing a fractal-camouflage Army Combat Uniform with a patrol cap. He had been looking down at his smartphone, so the visor of the cap covered his face, and I only noticed him when he looked up and his face was visible.
He had not been making any effort to hide. He was just standing around, against a backdrop of a grassy field that sloped up. The camouflage was good enough to completely fool the part of my visual cortex that looks for patterns and tells me when someone is nearby.
This incident is interesting by itself, but it is also related to the book Thinking, Fast and Slow, that I have been reading recently and highly recommend. The book surveys decades of research that show that much of our thought is automatic and unconscious, and subject to cognitive illusions that can hide the truth from us just like the optical illusions that let a cadet hide in plain sight.