When you save a page in Joomla using a front-end editor you may receive the "403 Forbidden" error if you have any links with the word "script" between the <a> and </a> tag. If you view the source of the error page, you'll see "<script> tag in POST".

This is because sh404sef checks URLs to make sure they are safe. This may block other HTML tags too.
To turn this security checking off:

1. Log in to Joomla's Administrator back-end
2. Go to Components > sh404sef
3. Select the Security tab
4. Change "Activate Security Functions" to No