The production error involves the contents of the Buy-a-Box promo pack that customers are eligible to receive with each box of Draft Boosters or Collector Boosters they buy. Each Buy-a-Box promo pack is supposed to contain one Athreos, Shroud-Veiled and one full-art foil Nyx land. Unfortunately, due to the production error, the lands in the promo pack are non-foil. To rectify this issue, WotC is currently printing up a new batch of replacement foil lands to send out to retailers.
Theros Beyond Death is scheduled to release on January 24 (see "Wizards of the Coast Previews 'Magic The Gathering' 'Theros Beyond Death'").