Captcha fails on comment previews

My users are reporting that it takes two tries to post comments. First answer is always rejected. This is true whether I have this set to Captcha math challenge or the new Captcha riddler, so it must be the Captcha mechanism itself.

I had the same problem and solved ist with the patch from http://drupal.org/node/142624

I am also having this problem -- it always takes two tries to get Captcha to pass. Unfortunately, not everyone tries a second time. I applied the patch captcha.module_2.patch, but it did not solve the problem for me. I am using PHP 5.2.1, just in case that matters.

the form store-module is necessary for captcha (http://drupal.org/project/form_store) - you use it?

Yes, form_store is installed and enabled for me. I have form_collect disabled, though -- does that make a difference? I had it enabled for a while to try to collect a Captcha point for my contact form, but then I turned it off after I got what I needed. I never turned off form_store, though.

I use form_collect - and perhaps it is necessary. My captcha-Point is the comment_form and I use the captcha with the riddler for this form.
I don't know much about the captcha-module - maybe you find a solution in other issues.

In Captcha 3.x, it doesn't fail, it just stays persistent on the form after clicking preview.... There's a related issue to disable the Captcha after previewing.