title: Magento Error while trying to order / cancel an order that has more than one item
body: <p>Hy</p>
  
  <p>After the onepage checkout, in Magento, I received the following email error message
  ...
  Payment transaction failed.
  Reason
  Mage registry key "_singleton/model/observer" already exists 
  Checkout Type
  onepage 
  ....  </p>
  
  <p>And on the admin page, when trying to delete this oreder, same error, again:</p>
  
  <pre><code>There has been an error processing your request
  
  Mage registry key "_singleton/model/observer" already exists
  
  Trace:
  #0 /app/Mage.php(223): Mage::throwException('Mage registry k...')
  #1 /app/Mage.php(478): Mage::register('_singleton/mode...', false)
  #2 /app/code/core/Mage/Core/Model/App.php(1316): Mage::getSingleton('model/observer')
  #3 /app/Mage.php(448): Mage_Core_Model_App-&gt;dispatchEvent('cataloginventor...', Array)
  #4 /app/code/core/Mage/Core/Model/Abstract.php(466): Mage::dispatchEvent('cataloginventor...', Array)
  #5 /app/code/core/Mage/CatalogInventory/Model/Stock/Item.php(787): Mage_Core_Model_Abstract-&gt;_afterSave()
  #6 /app/code/core/Mage/Core/Model/Abstract.php(319): Mage_CatalogInventory_Model_Stock_Item-&gt;_afterSave()
  #7 l/app/code/core/Mage/CatalogInventory/Model/Stock.php(210): Mage_Core_Model_Abstract-&gt;save()
  #8 /app/code/core/Mage/CatalogInventory/Model/Observer.php(809): Mage_CatalogInventory_Model_Stock-&gt;backItemQty('542', 1)
  #9 /app/code/core/Mage/Core/Model/App.php(1338): Mage_CatalogInventory_Model_Observer-&gt;cancelOrderItem(Object(Varien_Event_Observer))
  #10 /app/code/core/Mage/Core/Model/App.php(1317): Mage_Core_Model_App-&gt;_callObserverMethod(Object(Mage_CatalogInventory_Model_Observer), 'cancelOrderItem', Object(Varien_Event_Observer))
  #11 /app/Mage.php(448): Mage_Core_Model_App-&gt;dispatchEvent('sales_order_ite...', Array)
  </code></pre>
  
  <p>..and so on</p>
  
  <p>This only happends to orders that have more than one item. </p>
  
  <p>Any help will be appreciated, especially because the website is in production now.</p>
  
  <p>Many thanks</p>