Some are real, many have been "created" after the fact.
@KEY ISSUES Comics Which is why I said I am surprised CGC grades them at all, because if done well it is not really possible to know which is which. The "errors" supposedly exist because the foil was coming off on many copies due to the glue being poorly applied, or not applied at all, the foil has continued to flake off on copies over the years due to the cheap /poorly applied glue, this book obviously exists in ultra mass quantities and is cheap enough to try this with. I have personally seen copies where I could lift the edge of the foil off with just my fingertip. I am always leery when I see the copies with foil still remaining. The totally black ones where the foil looks more likely to never have been there at all are the only ones I would trust (and even then, you really just wouldn't know for sure).
-J.