If you’re visiting a web page over an SSL encrypted connection, you might sometimes get an annoying “mixed content” warning. In essence, the browser is telling you that yes, the page itself was delivered over HTTPS, but other bits were not, and that may or may not mean that you’re in danger.
From the developer point of view, having to build every URL so that the protocol is taken into account can be tedious. It might equally well be the case that your framework of choice already handles this. If so, good for you.
However, there’s another escape hatch not known to most people: protocol-relative URLs. You write the url without the protocol and following colon, and the client will choose whichever protocol was used to retrieve the page, like so: //server:port/path/to/resource.html
Not sure how often this is actually necessary, but there you go. :)