This happened twice today on my late 2010 MB Air (13"/2.13/4GB). The same thing also happened on my iMac once today. This was the first time it happened with my iMac since I bought it two years ago. I was just simply browsing the Internet, doing email, and word processing. The two most recent changes I made to both of them were installing dropbox and adding clicktoflash extension. Any idea what may have caused this? I'd appreciate your answers and advices.