Hello,
Sometimes in a moodleform you want to move to another page by javascript and a message appears saying "Are you sure...?"
The solution:
In your javascript code, use before call window.location:
window.onbeforeunload = null;
Best regards!
UPDATE 09/3/2015
P.D: Another option is editing the form and in the definition function add:
public function definition() {
$mform =& $this->_form;
$mform->disable_form_change_checker();
