Had to split my pennies by date and only do a certain amount a day to keep from going crazy. Lol Today was the day for 56 and 57. Noticed something in one of the 57s. The penny looks to be in really good condition, except for the date. The 5 is slightly faded, and the 7 is very faded. Thought about circulation, but, wouldn't that wear down the edges and, possibly the mint mark?
What is raised on a coin is recessed on the die. Sometimes recesses get clogged with junk and digits and letters fade away. It's not an error.