I was trying to load jquery in my site header. I was trying to use wp_enqueue_script(). But it was not working. No script was being loaded in header. I searched for solution. everybody says to use wp_enqueue_script(). But this was not working. finally I decided to deactivate some plugin for test. I when I deactivate this "WordPress Post Tabs" then problem solved. Now wp_enqueue_script() is working for me when this plugin is deactivated.