CAPTCHA test failed (unknown csid).

Please provide more info:
does registering work with the math CAPTCHA or image CAPTCHA?
does the reCAPTCHA also fail on other forms (e.g. comment form for anonymous users)?

I can confirm this bug. But it only happens if confirming the captcha using the RETURN key in Internet Explorer. For example, in Google Chrome it works. Therefore, I think it is a duplicate of: http://drupal.org/node/534168

at advseb in #2: I highly doubt this is a duplicate. The original poster does not mention internet explorer or using a return key. It's not because the symptom is the same ("unknown cid" message) that the problem is the same as in #534168: Submit with "return"/"enter" key in IE7 leads to failing CAPTCHA

I just committed a patch (http://drupal.org/cvs?commit=261468) for easier debugging the "unknown cid" problem,
Please install the latest 6.x-2.x-dev nightly built tarball and reproduce the problem, then look in the watchdog log for the related error message and report it here in full.

By the way, why are you changing this back to "support request"? For me this sounds much more than a bug and not a support request.

why are you changing this back to "support request"?

Because at this point, the original poster only described the occurrence of the "unknown csid" message. This message is raised by the CAPTCHA module itself and is a sign that there is something wrong with the site setup, not necessarily within the CAPTCHA module itself. Once we have more information about the problem (see e.g. #1), it could be clear that this is indeed about a bug in the CAPTCHA module. But at the moment, with the little information we have, it could just be a misconfiguration by the user or a problem caused by another module.

i'm getting this on a comment reply form, and if i look at $form_state, there is no entry for $form_state['clicked_button'] - is this a comment module problem?

@brad bulger in #7: I bet you're using Internet Explorer and using the return key to submit a form instead of clicking the submit button. Please join the discussion at #534168: Submit with "return"/"enter" key in IE7 leads to failing CAPTCHA

well i'm glad to know about that issue too, but i've since found that i was causing this problem in my theme - changing the #value of submit from 'Save' to 'Post comment'. undoing that stopped the problem. moving that change from a form theme function in the theme's template.php file to a form_alter function in a module avoided the problem and works fine.

nothing at all to do with captcha module code, in other words :)

This error also occurs, regardless of browser and regardless of key press, if you somehow get 2 rows with the same csid in the captcha_sessions table. Don't ask me how it happened, but it did. After repairing the table, the module functioned without error again.