Why I am getting the "The site's security certificate is not trusted!" error screen in Google Chrome 29?

You will see the "The site's security certificate is not trusted!" error screen in Google Chrome 29, because the server certificate provided by the Web site you are visiting is not signed by any trusted root CA defined in Google Chrome 29 as shown in this tutorial:

1. Visit https://www.cacert.org Web site with Google Chrome 29. You see the "The site's security certificate is not trusted!" error screen showing up with a message like this:

You attempted to reach www.cacert.org, but the server presented a 
certificate issued by an entity that is not trusted by your computer's
operating system. This may mean that the server has generated its own 
security credentials, which Chrome cannot rely on for identity 
information, or an attacker may be trying to intercept your 
communications.

You should not proceed, especially if you have never seen this warning 
before for this site.

2. Click "Proceed anyway" button on the error page. You see www.cacert.org page showing up.

3. Open the server certificate as shown in the previous tutorial. You see the server certificate is issued by "CA Cert Signing Authority".

4. Start "Certificate Manager" in Google Chrome 29.

5. Click "Trusted Root Certification Authorities" tab. You will not find "CA Cert Signing Authority" in the list.

See the picture below:

You can continue to use the web site by clicking "Proceed anyway", even its certificate is not trusted by the browser. You communication is still encrypted.

⇒ Connection Security Info in Google Chrome 29

⇐ View Website Server Certificate in Google Chrome 29

⇑ Managing Certificates in Google Chrome 29

⇑⇑ Google Chrome Certificates Tutorials