If you grab an item and try to switch it with another in the inventory quickly, it does not switch.

To replicate, just have 2 different items in inventory, grab one, and switch it with the other one. Do this very quickly, and try to move away from the slot at almost the same moment you switched it. If done correctly, the item will appear to switch for a split second, but go back to what you were previously holding. Some times it won't even appear to switch. This is much easier to replicate on a controller using the d-pad.