As this video shows, when I was looking up information on a product on Google, I found shortly afterward that Amazon knew about my Google search and put the product I was looking at in my "Recently Viewed" slot when I loaded up the retail site. This cross-site data leakage was due to the way the Invisible Hand extension works on Google's Chrome browser. The same issue happens on Internet Explorer when Invisible Hand is installed. Firefox is immune.
You can read the rest of this article at CNET News.com.