getErrorMessage triggered unexpectedly

Question

I have developed ajax addtocart functionality with a dropdown box that displays the item along with it's picture, price, etc. It also has 'close' and 'go to cart' buttons.

After the addition of an item no apparent error is raised. When I click the cart button on the main page the cart page does not display an error either. However, when I click the dropdown box's 'go to cart' button this is triggered on the next pageview (on the cart page).

               <?php if ($_rate->getErrorMessage()): ?>
                <ul class="messages"><li class="error-msg"><ul><li><?php echo $this->escapeHtml($_rate->getErrorMessage()) ?></li></ul></li></ul>

And, again, there is no error message that is displayed. It merely generated a list with the class of 'error-msg'.

Any idea as to why this might happen?

The button in question is:

<button id="checkout-btn" name="checkout-btn" type="button"><span><span><?php echo $this->__('Checkout') ?></span></span></button>

And the javascript function that handles it's onclick event is:

if(document.getElementById('checkout-btn').addEventListener){
    document.getElementById('checkout-btn').addEventListener('click',function(){
        setLocation('<?php echo Mage::helper('checkout/url')->getMSCheckoutUrl(); ?>');
    },false);
}
else if(document.getElementById('checkout-btn').attachEvent){
    document.getElementById('checkout-btn').attachEvent("on"+'click',function(){
        setLocation('<?php echo Mage::helper('checkout/url')->getMSCheckoutUrl(); ?>');
    })
}

I've even tried to remove all error messages in the session in the controller before the ajax response is sent back to the user:

Mage::getSingleton('checkout/session')->getMessages(true);
$this->getResponse()->setBody(Mage::helper('core')->jsonEncode($response));

Answer 1

The button that is used here uses a function that is more useful in other situations. In this certain one an error was thrown from current configurations.

Changing

getMSCheckoutUrl();

to

getCartUrl();

Seems to dissapate this error generation (which upon closer examination was a blank string of length 84 " ".