I noticed today that when trying to add more than what's in stock of an item on the store page, the main site's cart counter will still go up even though nothing gets added to the cart.
I had a figure that I had pre-ordered and saw that it was in stock in the store. I figured that it just hadn't shipped yet, but was curious how many were in stock. I used the exploit of adding as many of one item to your cart until it gets an error that you can't add anymore to see how many of an item are in stock and that's when I encountered the bug.
Recreation of events:
Adding more than what's in stock
Incorrect cart item counter on main site (store site cart counter didn't have anything)
Error page when clicking on the cart on the main site