When making a purchase (even for a free item) in the Marketplace, if the purchase hangs or never completes, and the player closes or restarts Minecraft, all minecoins will be gone from the account upon reentering the game with any connected device (does not have to be the same platform).

Repro:

• Start with an XBox Live account with a positive Minecoins balance 
• Start a purchase (tested with "Fall Buddies" pack)
• When the "Purchase In Progress" dialog appears, close Minecraft (close window or alt+f4)
• Re-open Minecraft on any device logged into the same XBox Live account
• Note Minecoin balance is now at zero