Every time I think I've found something cool, it turns out to be jewelry. What about this? It's a 1958 cent. The top of the "8" is raised, instead of indented. Was this a die break? It seems like the part that indents the 8, broke out of the die.
Some people call these a small 'cud'. I prefer to call them a die chip. I have found lots of closed letters filled.
Good News and Bad News. Good news is your finding them, bad news is this is really common to find filled dates and mintmarks. Also see alot of them with a die break in Liberty. Actually looks like it says Libierty. Keep looking! They are out there.