The client recently requested to change the text of the default Zend CAPTCHA error message to just Incorrect Code when a visitor tries to submit the form without entering the CAPTCHA code in the input box beneath the image.
That was pretty easy enough as Zend provides you the option to override the default error message.
You just need to add the following line to the parameters being sent when instantiating the Zend Captcha object.
'messages' => array('badCaptcha' => 'Incorrect Code')
After applying the line and testing it we get the desired message as can be seen below