US quarter mint error?

Hello,
Among my coins I encountered this one. Doesn't look like post-minting damage. Any thoughts on the possible reason? Should I keep it or just forget about it?

Taking into account that these quarters are copper-nickel plated copper, one would expect that filing (or even hammering) away that amount of detail would expose the copper core of the coin.
My odds are that this is not post mint damage.

Agree. Weak strike or partly obstructed die.
What does the opposite side look like ?

Re Mr. Midnight: the other side is as normal as can be (VF-XF).

I think it's a grease filled die.
like these: http://www.error-ref.com/?s=grease