Errors occur while making stamps, such as using wrong color, putting the original design upside-down or misdating, etc. As postal service policies require to destroy those error stamps before they come on market, stamps with error are rare and hard to find. Obviously, the rareness makes them even  more valuable. One of the well-known inverted stamps is called Jenny, which was  first issued on May 10 1918. The image is a Curtiss JN-4 airplane in the center of the design, and was accidentally printed upside-down.

Here are some error stamps for sale on eBay.