Researchers demonstrate privacy flaws in all major browsers, when browsing in privacy mode:
http://www.pcworld.com/article/202824/browser_privacy_modes_not_so_priva...
http://crypto.stanford.edu/~dabo/pubs/abstracts/privatebrowsing.html
These flaws are not exactly earth-shattering, but the report does call attention to the need for better testing and verification of private browsing mode in all browsers.