|FAQ - Cart Help Buttons - Extra Features - Inventory Control
Normally when the shopping cart displays an inventory control error, there will be a "Continue Shopping" button at the bottom of the error page.
They will then have the ability to correct the error, or simply continue shopping without correcting it. They would then see the error display again next time they added a product.
If you check this box, it will force your customers to correct the inventory error before it will allow them to continue shopping.