Jekyll does not like symbolic links. if you get error like
** ERROR: directory is already being watched! **
Directory: /Users/juha/github/jlento/presentations/_includes/slide-template/reveal/img
is already being watched through: /Users/juha/github/jlento/presentations/_includes/slide-template/img
MORE INFO: https://github.com/guard/listen/wiki/Duplicate-directory-errors
simply delete the symbolic links with
find _includes/slide-template -type l -delete_