After 65 years at Mad Magazine, comic artist Al Jaffee announced his retirement. Jaffee was best known for his Mad Fold-Ins, where folding the page would reveal a hidden message in the artwork. Plenty of examples can be found on the web. The problem is, they all show the before and after statically, side by side, which diminishes the magic (see here and here).
Source: The Mad Magazine Fold-In Effect in CSS, an article by Thomas Park.