Go in survival mode and use an iron pickaxe (it doesn't really matter what type of pickaxe you use but this is what I used), now mine away a small cave for you to try and recreate the bug.
This is where it gets difficult to explain- go to one wall of stone with a ceiling of stone above you, mine one block of ceiling which is in the corner of the top of the wall, BUT do it so you are away from the space so the item does not go into your inventory and the item 'Cobblestone' is in front of you and has landed on the ground.
The bug that I've noticed, is that no matter what server or single-player world I'm on, the item jumps back up to where it was mined and falls to the ground a second time. This happens also when you drop items, but its quite hard to see it so needs a keen eye to spot it. Hope this helps, if any questions just ask.
I have put a couple of screenshots to help with re-creation of bug-
First image- Just after the block is mined and item is still in air.
Second image- Just after the item lands, it travels back to where it was mined and falls to the ground again.