If when a client is trying to subscribe to a signup form on your web site and they get the above error then read on -
You can turn the security off and it will resolve the problem but that will open your mail list to spammers.
The official MailChimp reference
(https://us9.admin.mailchimp.com/lists/integration/embeddedcode?id=474357 - serch on page for the error)
How to fix it: Wait several minutes to see if you are able to successfully sign up with the email address or try a different browser. If your browser has an auto-fill setting, try turning it off—it can fill a hidden field that we include in the code to prevent spam signups.
We recommend keeping the hidden field in your signup form to prevent bots from signing up spam email addresses to your list. But it can be removed from your embed form code by deleting the following from the code on your site: