story from CNN:
A software developer in London has discovered a string of code that can carry over from your regular session into private mode, rendering privacy mode somewhat useless.
For example, let's say you use a regular browser to shop on Amazon and use Facebook. Then you launch privacy mode to visit a website that deserves more discretion, like a controversial blog.
If that blog uses the same ad network as Amazon or includes a Facebook "like" button, the advertisers and Facebook now know "Joe the Amazon shopper" and "Joe the Facebook user" is also "Joe the controversial blog reader."