Using the https:// URL for any of Puzzling.SE's chat rooms gives a security warning.
For example, in Firefox:
(Chrome isn't quite as in-your-face with it, but if you check the "View site information" $\require{enclose}\enclose{circle}{\sf{\tiny~\small i\tiny~}}$ it also tells you that "Your connection to this site is not fully secure".)
A quick check shows that amongst all the https:// Media objects there is one that is using the insecure http:// URL: http://cdn.sstatic.net/Sites/puzzling/img/bg_pattern.png?v=1 :
This background-overlay seems to be pulled in from the style sheet at
URL=https://cdn-chat.sstatic.net/chat/css/chat.puzzling.stackexchange.com.css?v=25d69e3b8357
I assume this inclusion of an insecure element on a secure page is unintentional, so posting this as a bug. P.SE folks, if you can confirm seeing this as well, it might not be a bad idea to say so here.