Move stack-able items in your inventory to "fast" and part of the stack will be placed wherever you were hovering your mouse.

If you're moving a stack-able item within your inventory of the inventory of chests/furnaces etc, it will place part of the stack where you hovered your mouse over with the stack. I think the issue is that the player is moving their mouse too "fast" and it registers that we place them in an area that we just hovered over with our mouse.