Well according to coin expert, Susan Headly

A doubled die coin is a coin that was struck from a die that had a partial or full doubled image on it. The doubling comes from mistakes in the die hubbing process, where more than one image is transferred from the hub to the coin die. The 1955 doubled die Lincoln Cent (see photo) is the classic example of a doubled die, and sparked the start of the error coin collecting hobby in the U.S. Although no other doubled die types exhibiting the remarkable degree of doubling seen on the 1955 cent have emerged from the U.S. Mint, noteworthy doubling has also been found on 1972 and 1995 Lincoln Cents.

I also checked out a great resource site for double die coin errors. So I will be referring to it. The site is called Wexlers Die Varieties .

I especially like this site because it shows and explains die errors for current coins like the Lincoln penny and others that came out in 2009.

So keep looking at that pocket change.