This is one of the Winners of the 2014 Best Illusion of The Year Contest. Enjoy!
Explanation: When we look at two pictures that are physically the same, they usually look the same. When they are different, they look different. Our illusions show the opposite: two images that are different but look the same – those are called “metamers”; – and two images that are identical but look different – we call those “anti-metamers.” Our main illusion mixes the two: it shows three images, two of which match with a third one mismatching. Viewers see one image as odd, but it’s one of the two identical images they see as different, an illusion we call “false pop out.”
(© 2014 Kimberley D. Orsten and James R. Pomerantz)
More visual illusions and brain teasers here: