After clicking on an item when in inventory, chest, crafting table, etc. and return to the game from the inventory screen, you loose whatever item or stack you clicked and held at the time. Rarely the item was dropped.
Comments 4
If you click on an item to move it to another inventory spot, and you leave the inventory screen and return to the game to play, the stack or item doesn't drop
Cannot reproduce neither in CSP nor in SMP.
What do you mean with "leave the UI" ? Close the game or "Save & quit to title" / "Disconnect" ?