At the top of the reverse side of the penny, the United States of America, it is slightly faded. The last S of States and the O of of are completely gone. While the other letters nearby also are more or less faded. I don't know if this is a grease error or what.
What is the weight of the coin. If it is less than 2.5grams it might be a thinned planchet? If not, then it is just as mentioned a grease issue. The STATES OF area is affected a lot on the Memorial cents from the metal movement during the strike.