Look closely to this screenshot:

This is a screenshot from a website, whose owner explains:

"I wanted to let you know that I took a screenshot of an Adsense unit run on my blog. (...) As you can see from the image, there is an error on one of the ads shown, and instead of showing the title of the ad, you see an URL ending with a number (15). The weird thing was that, everytime I was moving the mouse on that ad, the number was changing (16, then 17, 18 etc..) without any click made by me and even without refreshing the page or the adsense unit. So I thought that maybe (since I’m not sure) Google is also tracking the on mouse over effect on the adsense ads."

Via Blog.Outer-Court.com