Hey Dan,
It sounds like that the ajax registration is actually not working and the form is being posted with a normal post request. This is the fallback behavior. Most probably you have a js error oon the page that prevents the form to be sent with ajax. If you send it through ajax the validation will actually happen on the fly before sending. And if any error occurs when the form is sent it will be shown right where the form is and not in the standard error message container of your site.

Regards,
Daniel

We are experiencing a similar issue on a client site and tracked it down to pages on which the template included <jdoc:include type="message" />, which renders a <div> with id "system-message-container" for Joomla messages.

To ensure it wasn't a conflict with other components, I did a clean Joomla install and the issue was the same.
There are no jquery/javascript errors.
Removing <jdoc:include type="message" /> from the template index.php will resolve the issue but is not a realistic solution.

I've done a video demonstrating the issue at http://axses.com/vids/joomla-error.mp4

Please provide further guidance as to how to permanently resolve this problem.

We are using the latest versions of Joomla and CMC - Mailchimp.

Thank you.