The bug

When you place a recipe in the crafting field in the survival inventory and close the inventory in the exact same frame as you put the last item of the recipe into the crafting slots, a ghost item will be created upon reopening the inventory.

To reproduce

1. Limit your frame rate to 10 fps (or have a computer that is bad enough)
2. Open the survival inventory
3. Put a diamond block into the crafting field
4. Close the survival inventory in the same frame
5. Reopen the survival inventory
6. There will be nine diamonds in the crafting result slot. They will disappear when trying to pick them up, but you can transfer them into the normal inventory by pressing shift while clicking on them. In that case they will act like usual ghost items and disappear after a while.

Original description

When I placed (for example) redstone block into inventory crafting and quickly closed it and reentered it, then redstone block moved into inventory, when nine redstone stayed in crafting; I can grab it into inventory, however it vanished while I opened a chest.
Here is my amatour video (recorded with phone sigh) in which I'm showing this "copying": https://youtu.be/XT62JKFs4es