It's virtually impossible nowadays to have a new Magic: The Gathering product release without any advance information getting out to the public. That Wizards of the Coast managed to see Mystery Booster all the way through production and shipment to its world premiere in MagicFest Richmond, 7-10 November 2019 without spoilers or leaks is an astonishing feat all by itself!
At said world premiere, we learned that each Mystery Booster contains 15 cards. 14 of them appear to be previously released cards from throughout Magic's history up to the pr esent,from regular sets as well as specialty releases such as the Commander and Masters line. The oldest card we saw came from 2001's Odyssey, the newest from 2019's Throne of Eldraine. These cards all bear their own expansion symbols, but feature a small planeswalker symbol in the bottom left corner to mark them as Mystery Booster reprints.
The top card in every pack on the other hand is something never seen before. It is labeled as a "Test Card – Not for constructed play" and is printed to look like a regular card mostly covered by a white sticker, just like the ones Wizards of the Coast use internally to playtest new sets. The supposed test cards that made it into these boosters feature some outrageous, hilarious, or plain silly abilities such as one would expect to find in Unhinged or Unstable. For example, one creates land tokens, one changes who gets the first turn, and one—called "Sliv-Mizzet, the Hivemind"—grants all Slivers the abilities of Niv-Mizzet, the Firemind.
These cards introduce an element of additional randomness to Limited games. It is possible to get the equivalent of a Time Walk or Black Lotus, but there also are literal duds that tell you, for instance, to lose the game. If you enjoy such hijinks, Mystery Booster is the ideal product for you. Test cards aside, you effectively get the experience of a "Chaos Draft" —Limited with a mix of cards from lots of different sets and environments—just like players have been creating for themselves for years.
While we saw that boosters included up to two rare cards, and while some people have found a foil Temporal Mastery or a Mana Crypt from Eternal Masters in their pack, a lot of questions still remain. Where do these cards come from? How did Wizards determine the contents? And why?
And now, you can be part of the mystery too once Mystery Booster releases on 13 March 2020.