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."
Finalist of the Best Illusion of the Year Contest 2014
From Kimberley D. Orsten and James R. Pomerantz at Rice University, Houston, TX, USA