I've seen numerous examples of coins (via photos, no in person) that have one or more letters entirely missing ("IN GOD WE RUST", " IBERTY", etc.) and the usual explanation is that it is merely the result of a
Grease Filled Die and not, presumably, a more significant problem such as a cracked or otherwise damaged die.
From discussions around here, it seems as if these coins have little (if any) premium, though. Is that true? If so, why? Even if it's "only" caused by grease in the die, isn't it still a mint error and relatively rare? Or are there more of these around than I am assuming?