If you have the Q&A CAPTCHA enabled and you want to post something as a guest you will get the error:

Fatal error: Call to undefined method phpbb_captcha_qa::generate_code() in \includes\captcha\plugins\phpbb_captcha_qa_plugin.php on line 512

Because the phpbb_captcha_qa class is not extending phpbb_default_captcha, the generate_code() function is missing.
On the registration page I do not get this error, but the generate_code() function is also called in reset() so I think it should be there.