Putting bags inside of other bags
 

I came across this on my mule yesterday when I cleared his inventory out completely and then went to a backpack merchant and spam clicked to buy 8 backpacks. But there in the first backpack I had managed to buy more than the usual 8 and instead had backpacks inside my backpack.

The bag inside the bag can't be used for anything it seems, so i don't think this has any real exploit potential.