When trading, if you try to shift click the items from your inventory into the trading slot in a villager, it will instead move the items from your hotbar to your internal inventory, and vice versa.
How to duplicate:
1. Find a villager (or use a spawn egg)
2. Get the items required for the trade
3. After right-clicking the villager, try to shift click the items into the trading slots.
The items will not enter, and must be manually placed into the slots. This does not apply to removing items from the crafting grid, which works fine.
Confirmed.