Problem is that I turn the Captcha on by setting the Captcha Provider to Movable Type Default, and the captcha image with the input field are showing but when you submit a comment and the captch_code input field in empty or not the same as shown on the image it is still submitting the comment without getting any error.
Reported on Movable Type 4.3
That's very interesting!
Could you please supply me the URL of the project where this problem occurs?