I got lot of spam last Saturday even I enable recaptcha. I find following message on DEBUG log:

{Sat, 04 Aug 2012 12:23:55 GMT} Zenphoto v1.4.3.1[10590]
Backtrace: NOTICE: Undefined index: recaptcha_challenge_field in /home/pic/zp-core/zp-extensions/captcha/reCaptcha.php on line 46
captcha->checkCaptcha called
from postComment (functions.php [711])
from ThemeObject->addComment (classes.php [637])
from zp_handle_comment (functions-controller.php [146])
from require_once (controller.php [33])
from index.php [13]

Any hints?

Regards,
Chris Lee

First, you do know that Captcha is NOT a SPAM detection facility. It is a "human detector". Humans can SPAM.

I would guess that the error is being caused by a robot posting to your site. Presumably it did not provide the captcha parameters so the code faulted.