Players believe that because of a recent update (possibly yesterday’s Tavern Brawl), they’re missing cards from their Basic collection. Some players also have additional copies of Basic cards, which should be impossible. Since Basic cards are earned by playing the game and given to the player for free, they can’t be disenchanted or crafted.
Basic cards are one of the two main sets that never rotate out of Standard, the other being Classic. These cards act as a base for the game and are the building blocks for every expansion. Basic cards are important because they contain much of Hearthstone’s class identity.
Basic Mage cards like Arcane Intellect, for example, make you feel as though you’re manipulating your deck with magic by drawing more cards. Basic cards also make up the core of most competitive Standard decks. Losing your Basic cards is like losing the bottom row of your Jenga tower—without them, your collection will feel baseless.
Players who have experienced the bug say opening the collection manager nor opening a specific deck will fix the issue. For most, the bug eventually goes away on its own. But one user on the Blizzard forums reported that the cards vanished again shortly after returning.
If you experience this bug, the best thing to do is relaunch the game. If the missing or additional cards persist, try submitting a support ticket.