Steps to reproduce:
1. Buy out a villager's stock of any item. (until the red X appears over the arrow in the left sidebar.)

2. Pick up that item with the mouse and drag it over to the 3rd slot in the trading menu. 

3. Ensure that you still have enough to purchase the item if it were in stock.

4. Hovering over the 3rd, now empty, slot, left click as if you were picking up more of that item on the crafting menu.

The trade should still give you the item despite the fact that it's out of stock.

To my knowledge, this only works on stackable items.

UPDATE: I apologize for the lack of thorough testing before submitting. Here's a video of the bug.