I hate to leave a user with no way out if he/she decides not to complete a form, etc.
So, I've added a 'cancel' button to the end of the contact.php page right after the submit button:
<input type="submit" value="Send" id="sendme" title="Send your message" />
<input type="button" value="Cancel" onclick="window.location.href='../index.html'">
<!-- I knew where to send user, but otherwise use generic onClick="history.back()"> -->