If a dropper tries to move items into a container that can only hold certain types of items(Like a Furnace, Brewing Stand or Beacon) that don't normally fit(Like a stack of dirt in to a beacon) then the item stack will get removed and only 1 of the items will be left. To recreate: 1)Place a dropper facing in to a beacon 2)Put a stack of 64 dirt in to the dropper 3)Power the dropper 4)See that the stack disappears and only 1 block of dirt is left