An obvious mistake occurs if you could have known or understood that the price was much too low for the item that was offered. In other words, even if it wasn't immediately clear to you that there was a mistake here, the difference in price was so significant that there should at least have been reason for doubt. If there is any doubt about the correctness of the price, you as a consumer are obliged to make your own inquiries. If the difference in price is not significant, the we may not simply adjust the price of a product that has already been ordered.