The system has a limit as to how many files can be watched by a user. You can run out of watches pretty quickly if you have Grunt running with other programs like Dropbox etc.
So, if you want to watch a large number of files with Grunt like when using it with Wordpress you need to add this to increase the number of watches in your system, or else you may get the ENOSPC error.
I am using Linux (Elementary OS /Ubuntu )
echo fs.inotify.max_user_watches=524288 | sudo tee -a /etc/sysctl.conf && sudo sysctl -p
This will work with all versions of Debian based Linux.