Item Frame Duplication
If you place an item in an item frame, and then remove it two items comes out of the item frame, thusly duplicating the item.
Enchantments and usage is kept as they was, so it can be used to duplicate your really good items (or blocks) into multiple of that item (or block).
I found the bug in a realm I've joined, and it doesn't seem to behave the same in a local world. Not sure if the bug is related to which version was used to create the world, or being in the realm.