CPacketRecipePlacement can convert items

Based on 1.12 decompiled using MCP 9.40 PRE 1

The bug

The packet CPacketRecipePlacement can convert any item into a different one the player has in their inventory. This is possible by using the "from open container to inventory" item movement and specifying a destination which is already occupied by another item. Since the item type is not tested the item count is just increased, even if the moved item is a different one.

Ways to reproduce

Vanilla client - video

1. Fill your inventory with at least 5 stacks of wooden planks
2. Face down
3. Drop diamond on the ground
4. Spam click the recipe for crafting table
   → The diamonds will be duplicated when picked up

Modded client

1. Set the integer slot in CPacketRecipePlacement.ItemMove to 8
2. Prepare your inventory as shown in 2017-07-06_01.11.38.png
3. Press any item in the recipe book that does not involve the item in the crafting grid (ex. bed)
   → The items in slot 8 should have increased by the amount of items in the crafting grid
   Note: The 16 logs are client side