The Kissing Lincolns Coin, or the Inverted Double-Struck Penny, is a rare penny minted in December 1917 that was produced in a very small amount due to it being an error. One face of the coin had the image of Abraham Lincoln mirrored on it so it looked like they were kissing.
As part of a coin collecting book, this was the last one for Homer and Bart to collect. They went to auction for it and lost to Mr. Burns. They then asked for change for a nickel from him and he gave him the coin as change, happy that he had swindled them as he had not given them a full five cents before he realizes what he has done.