2008 Double Rim 5 cent piece
A number of 2008 5c pieces have been observed with double rims around the obverse and reverse - besides the normal rim there is a smaller, lower rim inside of the normal rim. They are not overly common however they are not rare. The double rim has led to speculation that they may be mules with a 1c obverse die, however the double rim appears on both sides of the coin, and no 2008 1c pieces were struck, so there would not have been any dies with which there could have been a mix-up.
The explanation that has been given is that some of the pre-rimmed blanks used to strike 2008 5c pieces had a slightly wider than normal rim, which, when struck, did not fully align with the rim on the die, thus leaving a slight overhang which manifests as a second rim.