Have you experienced a sudden increase in Adwords bid cost for your keywords? Some advertisers have seen a 10x and even 100x increase. The reason for this is a Google Adwords penalty for the site domain, but there is currently no way to confirm with Google that such a penalty has been imposed (and why). If this has happened to you, please have a look at our campaign to abolish hidden penalties. We want search engines such as Google to provide website owners with an answer to the question "Have I been penalized..?" Marc
Actually, Adwords does tell you that your costs will increase. They call it Quality Score. If your QS goes down, your costs will go up. Read more in the Adwords FAQ.