When use the middle mouse key to pick a block, it sometimes causes the old item to vanish from the inventory.

What I expected to happen was...

The old block to go into my inventory

What actually happened was...

The block did not go to my inventory, it disappeared. My inventory was not full. 

Steps to Reproduce:
1. Fill your hotbar completely full with items, then use the pick block key to select redstone dust on the ground. 
2. It isn't consistent, but you will find sometimes that the item previously in the hotbar dissapears from your inventory, instead of going into it.