It's a bit of imaginary situation and I think it's not that common but can happens and today someone asked me about that and I got curious but I have no easy way atm to test this so I thought in asking might someone already have tested. But the situation, imagine that you add a product that has 1 stock to the cart, you go to checkout and open the paypal window, in this point your order don't exist but you're already in paypal ready to pay, bu as far as I know the product is only reserved if the order exists and is in pending payment, thing that does not happens in paypal, so imagine that someone in the mean time that you are in paypal preparing your payment, bought that last stock that is not reserved because you didn't finished the order, you pay but the stock it's at 0 when the payment goes in, what happens in this case?
it might be confuse so in topics:
1- add a 1 stock product to cart
2- go to checkout and go to the paypal page to pay
3- other person bought that last stock and now that product has 0 stock
4- finish the payment
5- what happens? order created with a 0 stock product?
Thanks, hope it's understandable the situation :D