Nominal Item addItem function from app/code/core/Mage/Sales/Model/Quote.php

Nominal-addItem.php

<?php
/**
 * Adding new item to quote
 *
 * @param   Mage_Sales_Model_Quote_Item $item
 * @return  Mage_Sales_Model_Quote
 */
public function addItem(Mage_Sales_Model_Quote_Item $item)
{
    /**
     * Temporary workaround for purchase process: it is too dangerous to purchase more than one nominal item
     * or a mixture of nominal and non-nominal items, although technically possible.
     *
     * The problem is that currently it is implemented as sequential submission of nominal items and order, by one click.
     * It makes logically impossible to make the process of the purchase failsafe.
     * Proper solution is to submit items one by one with customer confirmation each time.
     */
    if ($item->isNominal() && $this->hasItems() || $this->hasNominalItems()) {
        Mage::throwException(
            Mage::helper('sales')->__('Nominal item can be purchased standalone only. To proceed please remove other items from the quote.')
        );
    }

    $item->setQuote($this);
    if (!$item->getId()) {
        $this->getItemsCollection()->addItem($item);
        Mage::dispatchEvent('sales_quote_add_item', array('quote_item' => $item));
    }
    return $this;
}