Magento’s compiler feature to place all the files in one place.
To remedy this issue, Magento core class should be included as follow.
<?php
if(defined('COMPILER_INCLUDE_PATH'))
require 'Mage_Customer_Model_Customer.php';
else
require 'Mage/Customer/Model/Customer.php';
class Mycompany_Mymodule_Model_Customer extends Mage_Customer_Model_Customer
{
...
}
Read More...