I would classify them as mistakes rather than errors. As such, I wouldn't place a premium on them and in fact I think the error would detract from the value of the item.

An error is usually something that was present on all units up until the error was noticed and then the items were either scrapped or shipped as is. the error was then fixed and two versions of the product made it to the market.

A mistake is just something that slipped by QC.

I know in the case of the coin posted that was not struck properly people may pay a premium for it but IMO that's still not an error. I've seen pictures of nickles struck with a penny die though and that would be an error. an error is like the upside down airplane because that "mistake" was noticed and only a certain amount of product with that mistake made it onto the market. the mistake was fixed and no more like it are produced.