This bug duplicates any item (blocks, items, spawn eggs, etc.) through a simple Creative mode exploit that creates an invisible item in your hot bar.

Steps to recreate bug:

1. Drop item (any item works)
2. Fly overhead over the item
3. Drop to the item while opening the Creative Menu
4. Open the Survival Inventory tab and move item to inventory.
5. Exit out the Creative Menu and continue to place blocks as normal.