Apparently there is a style of knitting that knits illusions into a sweater or scarf. If viewed from one angle you see one pattern, if viewed from another angle you see something different. Over at Heidi's Knitting Room I found the below.
ILLUSION KNITTING - ... It results in a fabric that has vertical stripes and as you turn it sideways, the pattern that you have knitted in will appear while the stripes seem to disappear - until you turn it again!
When you look at the sweater from the front, you see red and white vertical stripes and a shadow of what appear to be white hearts, but as you turn it sideways, you'll see a solid white background with red hearts. It's really amazing, I had never seen this before! Here is how it works: