I stumbled on this critical exploit (or rather smart technique) while I was visiting "How Web2.0 - Aware are you ??" They calculated a percentage based on the sites I have visited out of the 42 sites that were on their list. So how did they knew I visited these site ? Yeah they sniff my browser history !!
Actually this is a one lame example for practical usage of this new way of trackking users browser habits, which came into the play few days ago with Jeremiah Grossman's blog. However this bug is tracked in Mozilla before 4 years ago and kept there without getting caught to someone's eyes.
P.S. - It seems like Opera browser doesn't get affected by this exploit.