CAPTCHA session reuse attack detected error message when form is submitted
I had the same problem because I have an ajax file upload on the form. The solution is to patch the captcha module with the patch found here http://drupal.org/node/918856#comment-6353912 .
You just need to download the patch and put it in the same folder and run in command line: 

patch &lt; captcha-patch-10022012.patch


if using git, you can just use :

git apply --directory=your/module/derectory captcha-patch-10022012.patch