If a stay is reserved by someone else when it is still in your cart, it is no longer available to you. Because stays are booked on a first-come first-served basis, it is in your best interest to book quickly. KOA does validate the entire cart before booking on the checkout review page - so, in the off chance that a site is no longer available - we'll put a hold on all the bookings. You are then prompted with a message that your booking cannot be completed. You can either continue to book the remainder, or cancel and find another site.