math reports wrong correct value

On what form is that?

FYI: related issue for Drupal 7: #1067422: Captcha Incorrect answer

There was a change applied to the 7.x branch

http://drupalcode.org/project/captcha.git/commit/ef22105

The log entry says that it's connected to #1024370: CAPTCHA fails every time on some forms but I don't see a patch in that issue that matches, only a comment saying that it fixes the problem. It seems like that's a related issue to this one.

That same change is also mentioned as fixing this problem in #1067422-11: Captcha Incorrect answer which does seem like it's also a related issue.

The change does not appear to have ever been committed to the 6.x branch. We are hitting this same error, so I will try out the change to see if it helps. We can't reproduce the behavior at will to test for sure.

That change does not prevent this problem. This is still an issue with version 6.x at least. The same user in the same session will be able to use CAPTCHA successfully and then not, and then it will work again later.