Working on an early install of CentOS 6 and trying to update the system, the update failed. Even an attempt to update the ca-certificates failed. It turns out that the EPEL repository was the cause of this.

To work past the offending EPEL repository and upgrade the ca-certificates, the following command works.

yum upgrade ca-certificates --disablerepo=epel

Upon completion, a simple, yum update will work.

yum update

Source(s)
http://serverfault.com/questions/637549/epel-repo-for-centos-6-causing-error