From SA, fairly surprising--
and a funny article to go with--
http://www.somethingawful.com/articles.php?a=2858
Google has a product in testing called Google Web Accelerator. Its basically a caching proxy server that stores copies of the pages you visit or the images you view. When someone requests the same page, it tries to serve data out of the cache. This seems great, except that it means Google is likely storing private data that should NOT be available to the public. Some forum users have already gotten it to display private messages intended for other users.
and a funny article to go with--
http://www.somethingawful.com/articles.php?a=2858