Do you use Osx maverick?
checkout this: https://github.com/gruntjs/grunt-contrib-watch/issues/204
You need to upgrade node.js to 0.10.22 version:
# Check your version of Node.js. v0.10.20 will still be broken
node –v
# clean your node cache
sudo npm cache clean –f
# install node binary manager ’n’
sudo npm install -g n
# use said ’n’ manager to install latest stable version
sudo n stable
source: http://stephentvedt.com/blog/2013/11/16/grunt-js-watch-bus-error/