If you add a product from the manage product page, you select the category, then go to an add product page. If you enter data for the product, but don't fill in a sku and you have image lookup enabled, the customer side page throws and error value cannot be null input in several locations. When you save the product from the add product page, you think everything is OK, but when you click save you get directed to the Edit Product page and you have to save there in order to get the customer side message to go away and a default image to show. You have to click save on both pages.