For example, a simple product with custom options. in stock , qty (10). and no backorders (configuration / inventory / product stock options)
if customer add qty (6) of this product in cart and then click on 'edit' button from cart page, Magento redirect to checkout-cart-configure catalog-product-view product-id
here, if customer choose or change options, and keep the same qty (6) Click 'update cart' button Magento return error message (the quantity is not available).
So it seems that behind this update button, Magento add the new selection to the cart before delete the previous one..
Any fix for this would be appreciate