"Comment posted" message appears even when wrong CAPTCHA is added

Preconditions: Have the JCaptcha activated from Administration (with the selection of "Require unregistered users to solve a CAPTCHA when posting a comment on a page" from User & Rights > Rights) and Comments rights set for Unregistered users on a page.

Steps to reproduce:

1. Access the page with Comment rights as a guest (unregistered user)
2. Type a user name and a comment
3. Type a wrong CAPTCHA
4. Click on Add comment

Expected results: Comment isn't posted and the guest is asked to add the correct CAPTCHA.

Actual results: Comment isn't posted but a "Comment posted" message appears at the bottom of the screen.