Just experienced this recently. Actually this was my first experience since I am using OSX for the last 3 years.
After a while of surfing, Safari stops and the Mavericks as well. It forced me to push the reboot button. It never happened before in previous version of OSX. Done some googling research and it seems it happened also to others
Come on team, fix it!
Update on 25 Jan 2014.
I suspect the issue caused by Sophos Anti Virus. Remove it and replace it with Avast. Looks good so far.