If a player holds down control so that they would drop all of the stack of items that they are holding, and then press q (drop) and a number to select a different item than what they are holding, it will subtract one item from the both stacks, but on the ground will only display the item the player was originally holding. And when the player goes to pick it up they pick up the original item but not the second. If you reload the world it puts the lost item back in the inventory.