Jonty Hurwitz's three-dimensional objects look like amorphous blobs or a random assortment of objects. But when viewed from certain angles and often in mirrors, their intended impressions suddenly become clear. Hurwitz begins the creative process by scanning a three-dimensional object, then uses computational software to work out alternate physical forms. You can view many more examples at the link.
Hans Holbein's "The Ambassadors" is another example of this; it has a distorted skull (memento mori) painted into the foreground which can be seen properly by looking through a notch in the picture's frame, if I'm not mitaken. http://en.wikipedia.org/wiki/File:Hans_Holbein_the_Younger_-_The_Ambassadors_-_Google_Art_Project.jpg