Happened with quite a few of the Chinese Base Set cards, not just Koffing. Machop is the most well-known one, and for years I thought this type of error was exclusive to the Chinese Machop, but later on I learned there were more. I bought a few of those from a Chinese seller soon after I learned about it, so here an old picture of those (with the extra error parts highlighted in green:
And the TIL thread I created and still sometimes update is pretty similar to this one. It contains a lot of stuff I learned the moment I posted it and didn’t knew beforehand:
A snippet from that thread, in extension of what @Frosty.Puppy and @mrbigchip posted above about Unown spelling things within artworks: the Unown also spell things in other places:
