When you type an invalid email address in the email field of a form using the jQuery Validation Plugin, and then move focus to another field, it is supposed to show an error under the email field. 

It works as expected when the URL looks like this in the browser:
thanom.com/contact.php

However, when I try this same action on the exact same page where the URL is rewritten by the .htaccess file (i.e. thanom.com/contact/), then it does not show the error under the field anymore.

Thanks in advance for any replies.
Roxane