I get this from the error log all the time.
Cron Failure catalog_product alert:
---EXCEPTION---
exception 'Mage_Core_Exception' with message 'Invalid callback: Model for productalert/observer::process does not exist' in Mage.php:598
This is line 598: throw new Mage_Core_Exception($message); and this is the entire section of
/**
* Throw Exception
*
* @param string $message
* @param string $messageStorage
* @throws Mage_Core_Exception
*/
public static function throwException($message, $messageStorage = null)
{
if ($messageStorage && ($storage = self::getSingleton($messageStorage))) {
$storage->addError($message);
}
throw new Mage_Core_Exception($message);
}