What makes it an error coin?
Not really an error coin, but some of the sol noctis coins were not funded by the manufacturer, the polish mint. I've heard you can contact them about it and they will fund it or reimburse you, not sure though...
Wait, they are funded? I did not know that, which is a huge mistake on my part. I'll have to check if mine has been funded. Quite an expensive mistake on the companies part if they have to fund it right now.