This problem has been reported by one of our clients on a Drupal 6 site. They received many complaints from users that they were filling in CAPTCHAs correctly, but they were being told they were wrong. The number of complaints seemed to outweigh the possibility that it's just certain users who had mistyped a CAPTCHA.
We hacked the mollom module to intercept and log all mollom-served CAPTCHA images alongside the users' solutions. We did this by having the website request the CAPTCHA image from mollom, and serve it to the user.
We are finding that in more than 50% of the cases where mollom CAPTCHAs have been incorrectly solved, the image appears to be an exact match to what the user has entered, suggesting that mollom is getting it wrong.
I should note that we are using mollom on dozens of other sites, and none of the others have any reported issues.
I have no idea what the next step might be in identifying this problem, but there is definitely something wrong here so is there any way for us to find out what might be causing this?