The "back" button is not only in shippinginfo.php, but appears in about us, conditions, contact us, privacy, site map. etc.
When you place the mouse over the "Back" button, in the left corner of the brower you can see the link "http://www.best4future.com/page_not_found". When click the button, of course, you will be led to "Page Not Found"....
The same problem with my other webistes: https://www.theSbeauty.com,
https://www.roseforlove.com
On the tpl_about_us_default.php, I found the code liket this
<div class="buttonRow back"><?php echo zen_back_link() . zen_image_button(BUTTON_IMAGE_BACK, BUTTON_BACK_ALT) . '</a>'; ?></div>
</div>
Nothing looks wrong to me.....I turned on another computer and saw the same problem...
About "cookies", I clean them pretty frequently...