In survival mode, if you have an item in your inventory but not on your hotbar, using pick block on that item will cause the item to (correctly) move to your hotbar but to also create a duplicate version of the entity in another slot on your avatar.

Once you click on the item in the duplicated spot, the original item (ie. armour) is restored and the fake item disappears.

There is a video of this happening here:
https://www.youtube.com/watch?v=746_NNH_UfA