Actually, I've just been advised of a better option and that is the Invalid Slot Failure.
If you have created the set in your registry, you can click on the plus sign to add a card to the slot. If that card is not eligible for the slot, it will say "Your card does not fit in this slot. Think this is a mistake? Let us know >" Then click on the let us know link, and it will create an Invalid Slot Failure.
If there is no slot for the card you want to add, you can still use this feature, just choose another slot and upon review I will see that both the slot and the card need to be added.
The benefit of this is that once we review it and add the card to the slot (or add the slot and the card), it will add your card to your set automatically upon resolving the request.
Please see the images below: