Reason:
Mage_Catalog_Helper_Data::canApplyMsrp() method is implemented since Magento 1.6 version, attempts to use this method in earlier Magento versions, like Magento 1.5.1.0 or Magento 1.5.0.1 will result in fatal error.
I am assuming the error is caused at file location Mage_Catalog_Helper_Data::canApplyMsrp() in template/checkout/cart/item/default.phtml
The solution is to check if the method canApplyMsrp exists like below
Change
$canApplyMsrp = Mage::helper('catalog')->canApplyMsrp($_item->getProduct(),Mage_Catalog_Model_Product_Attribute_Source_Msrp_Type::TYPE_BEFORE_ORDER_CONFIRM);
To
if (method_exists(Mage::helper('catalog')->canApplyMsrp))
$canApplyMsrp = Mage::helper('catalog')->canApplyMsrp($_item->getProduct(),Mage_Catalog_Model_Product_Attribute_Source_Msrp_Type::TYPE_BEFORE_ORDER_CONFIRM);
I hope it helps..