先在Model文件夹下新建Nano_App_Model_Observer类文件，继承原有的类Mage_Captcha_Model_Observer：

app/code/local/Nano/App/Model/Observer.php

 * Rewrite Captcha Observer because error message should be given when captcha is not correct */class  extends Mage_Captcha_Model_Observer{}
 * Rewrite Captcha Observer because error message should be given when captcha is not correct */class  extends Mage_Captcha_Model_Observer{}
 * Rewrite Captcha Observer because error message should be given when captcha is not correct
 */
class  extends Mage_Captcha_Model_Observer
{
}

再修改app模块的配置文件config.xml,使得复写生效

app/code/local/Nano/App/etc/config.xml

<models>
    <nano_app>
        <class>Nano_App_Model</class>
    </nano_app>
    <captcha>
        <rewrite>
            <observer>Nano_App_Model_Observer</observer>
        </rewrite>
    </captcha>
</models>