Arguably ever single coin in existence is unique in one way or another, the same as every living person/animal is unique.
I own a lunar mule, I consider this a true error/limited edition variation as admitted by rm.
Then there are people who insist on calling things like the 'standing on the shoulder of gants' an error and then hike up the price when selling the coin to others. I would consider this most likely a worn/degraded die where the 'i' in giants is missing as the spacing is identical to that of a 'giants' coin. I don't ask for a premium for my worn 'fair/fine' coins with some of the wording missing so why should they? I'm also sure that these same sellers will be most reluctant to buy their so called error coin at a premium. I think they are simply there to rip people off and give coin collectors a bad name. It just seems that for some it's like every single coin that comes into their possession is a unique error coin that deserves a premium.
any opinions?
HH