Using a really fast clock that controls a trapdoor (any type) with items on top causes items to be falling below the trapdoor but they are ghost items. cant be interacted, likely a client side error / view. And when the items take at least 5-10 seconds of air travel, they glitch back onto the trapdoor, and it loops going down and up.
This report is invalid as your game was modified when experiencing this issue. The asterisk displayed in the title of the window indicates that you're running a non-vanilla instance of Minecraft.