Sometimes you receive an error "The content on this page has either been modified by another user, or you have already submitted modifications using this form. As a result, your changes cannot be saved." .
The above error means
1) Either the problem is related to two users editing the same node
2) Or it can be related to max_input_vars in your php.ini
1) Clear cache of your browser
2) Edit php.ini file. Usually its located on /etc/php5/...