I agree Jim, it all has to do with rarity. A slight error that is within quality control or slightly out may be common where the extreme errors would be pulled in the manufacturing process making them more scarce. There is a big grey area. I also agree with Archie in that a slight error just represents poor workmanship/quality control and are allowed to go out for use, the stamp with the upside down airplane is an extreme error that IMO was never intended to be released, making it rare and valuable. I see currency all the time with an 'error' so slight you have to examine it close to even find it, that to me is of little to no premium value, but an upside down serial number is.