The first Mysterious card set appeared in June, 2016. This set foreshadowed the Summer Sale 2016. The second set appeared in December, 2016. This set foreshadowed the Holiday Sale 2016. Crafting badges from any other game granted one mystery card, replacing coupons you would get regularly. These cards are not craft-able into a badge, but instead will be replaced with a corresponding card once it's sale begins.
Note: If any of the cards in the set that you craft into a badge is blocked from being sold or traded, the crafted card will also be blocked from being sold or traded.