Recently I had to redo something at work. It involved a servlet on Tomcat that accepted requests from external users (Connection A), which in turn connected via SOAP to a Web Service on the same box (Connection B). If Connection A used a secure connection, Connection B had to use one as well, otherwise my servlet would keep throwing connection errors and logging that the connection was reset. When I used an unsecure connection for both, they worked fine, so it was a certificate issue in the lab.
In general, I understand why mixed-content connections are discouraged or just refused by the client. But given that this was explicitly sent over localhost, it struck me as a bit of a hassle. So, if a connection is over localhost, why is this still neccessary?