I happened to notice that, while browsing SE over HTTPS, a blank ad-shaped spot sometimes appeared in the sidebar. Looking more closely, I found the following piece of HTML in it:
http: script URL, which naturally gets blocked as insecure when viewing the page over HTTPS.
Sure enough, loading the script with the protocol changed to
https: brought up a nice ad for a scifi.SE question in place of the empty box:
I like these pretty ads, and I wish I could see more of them, even over HTTPS. Removing the
http: prefix from the script URL to make it protocol-relative ought to fix it.
(Ps. Sorry, I forgot to take screenshot of the empty space. It looked the same as above, except with blank space where the ad is.)
Edit: OK, I spotted it again, and this time I managed to grab a screenshot of the broken ad space in the wild, along with its HTML code as seen in the dev tools: