What kind of error is a coin with a letter filled in say the "B" in liberty is filled in completely with metal? Or for example the "9" in 1919 is filled in completely on a wheat penny from 1919? Are these the same type of error? Are they worth anything?
It's caused by the die chipping away and leaving that void in the die. Worth anything? At least one cent. Some have more value to some. Check e-bay there's loads of them there.
Or a die chip if the filled in part of the device is raised. Greasers will be flat, almost to the level of the field.
What the OP described is a chipped die, not a grease-filled die. A greased-filled die would caused a raised device to be struck incomplete, not with extra metal.
Try to imagine that you're looking at the face of a die. The recessed parts of the devices create the raised areas of the coin. The center of any letter or number is created by a small post of metal on the die. If this post breaks away, the metal would flow into the void, thus filling the center of the letter or number. Chris