what if an item sells out whilst another customer is checking out?

I have a quick question, if one customer (customer A)is just about to checkout an item in their cart and hits the checkout button and another customer (customer B) at the same time decides to add that item to their cart (that is not marked as sold out yet)in the same moment and its the last item what happens?

:cool:

Once the last item has finished checkout, another item won't be able to finish checkout. I don't know exactly when the second customer's shopping cart will reflect the situation; probably the next time they refresh their page, but possibly not until they try to check out.