AdTrafficQuality - 2:10 am on Jul 3, 2013 (gmt 0)
We at Google are always working to improve and refine our automatic filtering to better detect invalid ad traffic. As a result, you may see a change in the number of valid clicks you are receiving. Invalid clicks can come from a variety of sources including bot traffic and/or publisher implementations that encourage accidental clicks. If you have recently purchased any traffic, try temporarily disabling the traffic. If your ad units may confuse users in any way, we recommend changing your implementation in order to clearly delineate ads from site content, including navigation controls. You may also opt out of "Enhanced Text Ads" via the "Allow & block ads" category in the AdSense front end.
Ad Traffic Quality @ Google