Am not sure if I have clicked something or what but the display looks a bit vague/hazy everytime this happens. This is different from not responding coz there is no rainbow/umbrella cursor. And I can still use Safari by openning another tab or window. When this happens, that particular tab which "froze" cannot be used in the sense that I cannot go to another website. I have to close that tab or open a new one.
MacBook Pro (13-inch Late 2011), Mac OS X (10.7.3)