It is kinda hard to describe, but basically sometimes the item in my hand is dropped and disappears. It is then replaced with another item from my inventory in survival mode (not sure on this one). Reloading the world causes the dropped item to appear in front of me on the floor. I have no clue exactly what happens so that might be wrong but all I know is I keep dropping my pickaxe and having to reload my survival world for it to reappear.