The WaPo has an interesting article about that ugly dress and the debate that raged around the globe and kept us distracted for much more important things. The article provides numerous examples of optical illusions that make us see things differently than they really are, but this one was my favorite.
In this image, the surface of each half of the object are an identical color. You can test this by holding your finger over the middle of the image where the to halves meet. Mind. Blown.