An error card is something that wasn't suppose to be on the card, or has nothing to do with that particular card....like swatches or printing for that matter
Well we all know the person who purchased it - why don't you ask her?
I was looking for variants at the time and was emailing Richard (cardvault) about specific variants and I brought up the Igor costume card that had the blue swatch. I had only seen one on ebay scousse691, was the seller. I questioned the legitimacy of the card because there is no visible blue in the costume. He then stated that he had one as well. I then emailed Tom Breyer and asked him if he knew anything about blue being in the costume or if he knew of any errors in production. Tom then contacted artbox and artbox said it had to be an error in production.
After several emails back and forth between myself, Richard and Tom; I guess Richard decided to listed the card as BIN on ebay, possibly to see if I or someone else would pay that much for it. I bought it out of curiosity. It was of course the dark blue material from the beauxbatons uniform. In light of what I have paid for some of my "variants" - it wasn't that bad of a deal. I believe the one scouse sold went for around 210 USD.
Think its interesting that there is a beauxbatons card with the Igor material. Wouldn't be interested in selling it would you?

I consider my card an error, not a variant. I bought it specifically because it was an error and there were not many of them; only 2 that I know of. I agree its not required for the set to be a master set.
