Are we funning with you here? The two shades of grey obviously are not the same because there is a clear boundary between them! However, get close to the screen and run your eye vertically up or down. You will see that each shade of grey matches the overall background of the drawing, with NO BOUNDARY! How is it possible for there to be a visible boundary in some places and not others?

Find similar illusions here. Copyright A.Kitaoka, 2002

How about a variation, the Koffka illusion? The image at the left is the same as the image at the right, except that on the left one side has been offset upward. The illusion that the C-shape is a different shade of grey on one side than the other is very strong in the left-hand version, and disappears in the right-hand version. One of many interesting variations of standard illusions, by Catherine L. Palmer... check out the original here. For an animated version of the illusion, click on either of the images.